SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 13D/A

Under the Securities Exchange Act of 1934

(Amendment No. 6)*

Cornerstone Therapeutics Inc.

(Name of Issuer)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

21924P103

(CUSIP Number)

David B. Clement

Smith, Anderson, Blount, Dorsett, Mitchell, & Jernigan, L.L.P.

Wells Fargo Capitol Center, Suite 2300

Post Office Box 2611

Raleigh, North Carolina 27602-2611

(919) 821-6754

(Name, Address and Telephone Number of Person Authorized to Receive Notices and Communications)

September 15, 2013

(Date of Event which Requires Filing of this Statement)

If the filing person has previously filed a statement on Schedule 13G to report the acquisition that is the subject of this Schedule 13D, and is filing this schedule because of Rule 13d-1(e), 13d-1(f) or 13d-1(g), check the following box:  ¨

Note: Schedules filed in paper format shall include a signed original and five copies of the schedule, including all exhibits. See Rule 13d-7 for other parties to whom copies are to be sent.

The information required on the remainder of this cover page shall not be deemed to be “filed” for the purpose of Section 18 of the Securities Exchange Act of 1934 (“Act”) or otherwise subject to the liabilities of that section of the Act but shall be subject to all other provisions of the Act (however, see the Notes).

Schedule 13D/A

This Amendment No. 6 (this “Amendment”) is being filed in connection with the entry by the Reporting Persons (as defined below) into the Merger Voting Agreement (as defined below). This Amendment amends and restates in its entirety the Report on Schedule 13D, originally filed on May 12, 2008, as previously amended by Amendment No. 1, filed on November 7, 2008, Amendment No. 2, filed on May 18, 2009, Amendment No. 3, filed on August 4, 2009, Amendment No. 4, filed on December 21, 2010 and Amendment No. 5, filed on April 5, 2012 (as amended and restated, the “Schedule 13D/A”).

Item 1. Security and Issuer.

This statement on Schedule 13D/A relates to the common stock, par value $0.001 per share, of the Issuer. The address of the Issuer’s principal executive offices is 1255 Crescent Green Drive, Suite 250, Cary, NC 27518.

Item 2. Identity and Background.

This Schedule 13D/A is being filed jointly pursuant to Rule 13d-1(k)(1) on behalf of Cornerstone Biopharma Holdings, Ltd., an Anguilla company (“CBPHLtd”), and Craig A. Collard, a citizen of the United States. The principal business of CBPHLtd is investing in the stock of the Issuer. Mr. Collard is (i) the Chief Executive Officer and Chairman of the Board of the Issuer and (ii) the President, Chief Executive Officer, Director and beneficial owner of 100% of CBPHLtd. CBPHLtd and Mr. Collard are collectively referred to herein as the “Reporting Persons”.

The business address of CBPHLtd and Mr. Collard is c/o Cornerstone Therapeutics Inc., 1255 Crescent Green Drive, Suite 250, Cary, NC 27518.

The Reporting Persons have not, during the five years prior to the date of this Schedule 13D/A, (i) been convicted in a criminal proceeding (excluding traffic violations or similar misdemeanors) or (ii) been a party to a civil proceeding of a judicial or administrative body of competent jurisdiction, as a result of which the Reporting Persons were or are subject to a judgment, decree, or final order enjoining future violations of, or prohibiting or mandating activities subject to, federal or state securities laws or finding any violation with respect to such laws.

Please see Schedule I for information pertaining to the executive officers and directors of CBPHLtd.

Item 3. Source and Amount of Funds or Other Consideration.

Chiesi Investment in the Issuer and Initial Purchase from CBPHLtd – May 6, 2009

On May 6, 2009, the Issuer and Chiesi Farmaceutici S.p.A. (“Chiesi”) entered into a Stock Purchase Agreement (the “Stock Purchase Agreement”), pursuant to which the Issuer agreed, subject to the terms and conditions set forth in the Stock Purchase Agreement, to issue and sell 11,902,741 shares of the Issuer’s common stock, par value $0.001 per share (“Common Stock”), to Chiesi (the “Issuer Stock Sale”). The Stock Purchase Agreement provided that, in exchange for the shares to be issued to Chiesi, Chiesi would (i) grant the Issuer an exclusive ten-year license to distribute and market Chiesi’s Curosurf^® product in the United States and (ii) pay the Issuer $15,465,075 in cash. The Stock Purchase Agreement also contemplated that the Issuer’s certificate of incorporation and bylaws be amended to incorporate certain corporate governance provisions consistent with the terms of the Governance Agreement described below.

Concurrently with the execution and delivery of the Stock Purchase Agreement, Chiesi and two stockholders of the Issuer, including CBPHLtd, entered into a separate stock purchase agreement (the “Stockholders Stock Purchase Agreement”), pursuant to which the two stockholders agreed, among other things, to sell to Chiesi 1.6 million shares of Common Stock owned by such stockholders, with CBPHLtd selling 1,250,000 at $5.50 per for aggregate proceeds of $6,875,000 (the “Initial Stock Sale”). Following the closing of the Initial Stock Sale and the closing of the Issuer Stock Sale on July 28, 2009, Chiesi owned approximately 51% of the Issuer’s outstanding Common Stock on a Fully Diluted Basis (as defined in the Stock Purchase Agreement).

In connection with and concurrently with the execution and delivery of the Stock Purchase Agreement, the Issuer, Chiesi and, solely with respect to certain sections identified therein, certain stockholders of the Issuer, including CBPHLtd and Mr. Collard, entered into a Governance Agreement (the “Governance Agreement”), which set forth certain rights and obligations of the Issuer, Chiesi and such stockholders concerning, among other things, certain corporate governance matters, the voting of Chiesi’s shares of Common Stock, certain limitations on future acquisitions and dispositions of shares of Common Stock by Chiesi and certain rights of first offer to distribute and market the other party’s products. The Governance Agreement became effective upon the closing of the Issuer Stock Sale.

In connection with and concurrently with the execution and delivery of the Stock Purchase Agreement, the Issuer, Chiesi and certain stockholders of the Issuer, including CBPHLtd and Mr. Collard, entered into a Stockholders Agreement, as amended (the “Stockholders Agreement”) pursuant to which the stockholders agreed not to sell or otherwise transfer a number of shares equal to approximately 80% of the shares of Common Stock held by them as of May 6, 2009 (the “Covered Shares”), subject to certain exceptions described in the Stockholders Agreement. In addition, the stockholders agreed they would not, directly or indirectly, acquire or offer to acquire any shares of Common Stock, subject to certain exceptions described in the Stockholders Agreement. The Stockholders Agreement also provided that beginning on the date on which the restrictions on transfers by the stockholders of the Covered Shares lapsed and for a 30 day period thereafter, Chiesi had the option, exercisable in whole but not in part on a single occasion, to acquire all the stockholders’ Covered Shares, at a price per share of $12.00 (subject to adjustment for any stock split, stock dividend, reverse stock split or similar adjustment). Each stockholder also agreed, subject to certain conditions, that at any meeting of the stockholders of the Issuer called to consider a transaction in which Chiesi or its affiliate would acquire all the outstanding capital stock of the Issuer, the stockholder would vote all shares of Common Stock owned by such stockholder at the applicable record date set for such meeting in the same proportions that the shares of Common Stock owned by the other stockholders of the Issuer (other than Chiesi and its affiliates) were voted on such matter. The Stockholders Agreement became effective upon the closing of the Issuer Stock Sale.

In connection with and concurrently with the execution and delivery of the Stock Purchase Agreement, the Issuer and Chiesi entered into a Registration Rights Agreement (the “Chiesi Registration Rights Agreement”), pursuant to which the Issuer agreed to provide registration rights to Chiesi with respect to the shares of Common Stock to be acquired in the Issuer Stock Sale. Under such agreement, following the Blackout Period, Chiesi was entitled to require the Issuer to file with the SEC certain registration statements under the Securities Act of 1933, as amended, (each a “Demand Registration”) with respect to the resale of the shares of Common Stock acquired pursuant to the Initial Stock Purchase Agreement and the Stock Purchase Agreement up to four times, and to include its shares of Common Stock in any registration the Issuer proposed for its own account or for the account of one or more of its stockholders.

In connection with and concurrently with the execution and delivery of the Stock Purchase Agreement, the Issuer and the stockholders of the Issuer who entered into the Stockholders Agreement also entered into a Registration Rights Agreement (the “Stockholders Registration Rights Agreement”) substantially similar to the Chiesi Registration Rights Agreement. Under such agreement, such stockholders were entitled to two Demand Registrations during the Blackout Period and three Demand Registrations thereafter. The stockholders also had the right to include their shares of Common Stock in any registration the Issuer proposed for its own account or for the account of one or more of its stockholders.

In connection with and concurrently with the execution and delivery of the Stock Purchase Agreement, the Issuer and Chiesi entered into a Voting Agreement (the “Chiesi Voting Agreement”), pursuant to which Chiesi agreed to vote all of its shares of Common Stock in favor of the approval and adoption of the proposed amendment to the Issuer’s certificate of incorporation.

In connection with and concurrently with the execution and delivery of the Stock Purchase Agreement, the Issuer also, on May 6, 2009, entered into a voting agreement with Chiesi and certain stockholders of the Issuer named therein, including CBPHLtd and Mr. Collard (the “Stockholders Voting Agreement”), solely with respect to Section 2(b) thereof, which provided that the Issuer would not, and was unconditionally instructed not to, permit on its books and records transfers by, issue new certificates to or record any vote of such stockholders, unless such stockholder had complied with the terms of the Stockholders Voting Agreement. Pursuant to the Stockholders Voting Agreement, the stockholders named therein granted to Chiesi irrevocable proxies over the shares of Common Stock owned by them and agreed to vote the shares of Common Stock owned by them in favor of the Issuer Stock Sale and approval and adoption of the proposed amendment to the Issuer’s certificate of incorporation, subject to the terms and conditions of the Stockholders Voting Agreement.

On July 28, 2011, the Governance Agreement terminated pursuant to its terms; however, certain of the other agreements discussed above continue to have ongoing obligations.

Chiesi Additional Purchase of Shares from CBPHLtd – December 16, 2010

Pursuant to the Governance Agreement while it was in effect, Chiesi was permitted to make additional purchases of the Issuer’s common stock to the extent necessary to maintain its beneficial ownership of the Issuer’s common stock at 51% on a Fully Diluted Basis (as defined in the Governance Agreement). As a result of the issuance of additional equity awards under the 2004 Plan (as defined below) following the closing of the Initial Stock Sale, Chiesi’s beneficial ownership on a Fully Diluted Basis had dropped below 51%. Accordingly, Chiesi approached CBPHLtd and another stockholder regarding their willingness to sell additional shares to Chiesi in a private transaction. The parties agreed that a private sale was in the best interests of the parties, the Issuer and the Issuer’s other stockholders in that it would avoid potential short-term market distortions that could have occurred had Chiesi sought to purchase the shares in the open market.

Accordingly, on December 16, 2010, CBPHLtd and the other stockholder sold an additional 385,000 and 65,000 shares, respectively, of the Issuer’s common stock to Chiesi pursuant to a stock purchase agreement dated December 16, 2010 (the “December 16, 2010 Stock Purchase Agreement”) at a price of $6.02 per share, for aggregate proceeds of $2.7 million. The agreed-upon sales price was determined based on the average closing price of the Issuer’s common stock during the twenty trading days prior to the date of the sale.

Chiesi Merger Agreement – September 15, 2013

On September 15, 2013, the Issuer entered into an Agreement and Plan of Merger (the “Merger Agreement”) with Chiesi and Chiesi U.S. Corporation (“Chiesi US”), a Delaware corporation that is a wholly owned subsidiary of Chiesi, whereby Chiesi US would merge with and into the Issuer, with the Issuer surviving the merger as a direct wholly owned subsidiary of Chiesi (the “Merger”). In the Merger, each share of the Issuer’s common stock that is issued and outstanding as of the effective time of the Merger, except for treasury stock, dissenting shares and shares held by Chiesi or its subsidiaries, will be converted into the right to receive $9.50 in cash, without interest and subject to deduction for any required withholding taxes.

In connection with the Merger Agreement, on September 15, 2013, CBPHLtd and Mr. Collard entered into a voting agreement with the Company, Chiesi and Chiesi US (the “Merger Voting Agreement”) under which they have agreed to vote their shares of the Company’s common stock in favor of the Merger.

The descriptions of the Stock Purchase Agreement, the Stockholders Stock Purchase Agreement, the Governance Agreement, the Stockholders Agreement, the Chiesi Registration Rights Agreement, the Stockholders Registration Rights Agreement, the Chiesi Voting Agreement, the Stockholders Voting Agreement, the December 16, 2010 Stock Purchase Agreement and the Merger Voting Agreement do not purport to be complete and are qualified in their entirety by the following, which are incorporated herein by reference: (i) the Stock Purchase Agreement, which is referenced herein as Exhibit 10.01; (ii) the Stockholders Stock Purchase Agreement, which is referenced herein as Exhibit 10.02; (iii) the Governance Agreement, which is referenced herein as Exhibit 10.03; (iv) the Stockholders Agreement, which is referenced herein as Exhibit 10.04, as amended by Exhibit 10.09; (v) the Chiesi Registration Rights Agreement, which is referenced herein as Exhibit 10.05; (vi) the Stockholders Registration Rights Agreement, which is referenced herein as Exhibit 10.06; (vii) the Chiesi Voting Agreement, which is referenced herein as Exhibit 10.07; (viii) the Stockholders Voting Agreement, which is referenced herein as Exhibit 10.08; (ix) the December 16, 2010 Stock Purchase Agreement, which is referenced herein as Exhibit 10.10; and (x) the Merger Voting Agreement, which is attached hereto as Exhibit 10.11.

Item 4. Purpose of Transaction.

The original acquisitions of securities of the Issuer by the Reporting Persons were for investment purposes or in connection with the Issuer’s equity compensation arrangements. Subject to, among other things, the Issuer’s business prospects, prevailing prices, market conditions, and the terms and conditions of the agreements described in Item 3 above, the Reporting Persons may purchase or dispose of additional shares of common stock and/or other securities of the Issuer from time to time in the open market, in privately negotiated transactions, or otherwise.

Except as set forth in this Schedule 13D/A, the Reporting Persons have no plans or proposals which would relate or result in any of the matters set forth below:

Item 5. Interest in Securities of the Issuer.

(a) and (b). As of the date of this Schedule 13D/A:

(i) CBPHLtd has shared power to vote or to direct the vote of, and shared power to dispose or to direct the disposition of, 1,567,225 shares of common stock of the Issuer, which represents approximately 5.8% of the Issuer’s outstanding common stock;

(ii) Craig A. Collard may be deemed to have sole power to vote or to direct the vote of, and sole power to dispose or to direct the disposition of, 45,000 restricted shares of common stock and 606,113 shares of common stock of the Issuer underlying options exercisable within 60 days, which combined represent approximately 2.4% of the Issuer’s outstanding common stock. Mr. Collard acquired these restricted shares and options pursuant the Cornerstone BioPharma Holdings, Inc. 2005 Stock Incentive Plan and the Cornerstone Therapeutics Inc. 2004 Stock Incentive Plan (the “2004 Plan”). Copies of these plans, as well as the forms of restricted stock and option awards made thereunder, are referenced herein as Exhibits 10.12 through 10.17. Mr. Collard owns 100% of CBPHLtd, and consequently may be deemed to be the beneficial owner of any shares deemed beneficially owned by CBPHLtd.

As discussed in Item 3 above, the Reporting Persons have entered into various agreements with respect to the Common Stock beneficially owned by them. Such information is incorporated into this Item 5 by reference.

(c). Except as described herein, none of the Reporting Persons has effected any transaction in the Issuer’s common stock during the past 60 days.

(d). Not applicable.

(e). Not applicable.

Item 6. Contracts, Arrangements, Understandings or Relationships with Respect to the Securities of the Issuer.

As discussed in Item 3 above, the Reporting Persons have entered into various agreements with respect to the Common Stock beneficially owned by them. Such information is incorporated into this Item 6 by reference. Except as described in this Schedule 13D/A, there are no contracts, arrangements, understandings or relationships (legal or otherwise) among the persons named in Item 2 and between such persons and any person with respect to any securities of the Issuer, including but not limited to transfer or voting of the securities, finder’s fees, joint ventures, loan or option arrangements, puts or calls, guarantors of profit, division of profit or loss or the giving or withholding of proxies.

Item 7. Material to be Filed as Exhibits.

SIGNATURES

After reasonable inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

EXHIBIT INDEX

SCHEDULE I

Executive Officers and Directors of Cornerstone Biopharma Holdings, Ltd.

Exhibit 10.11

VOTING AGREEMENT

This VOTING AGREEMENT (“Agreement”) is made as of September 15, 2013, among Chiesi Farmaceutici S.p.A., an Italian company (“Parent”), Chiesi U.S. Corporation, a Delaware corporation and wholly owned subsidiary of Parent (“Merger Sub”), Cornerstone Therapeutics Inc, a Delaware corporation (the “Company”) and each of the undersigned stockholders of the Company (each a “Stockholder” and together, the “Stockholders”).

RECITALS:

WHEREAS, concurrently with the execution and delivery of this Agreement, Parent, Merger Sub and the Company are entering into an Agreement of Plan of Merger of even date herewith (as the same may be amended from time to time, the “Merger Agreement”), pursuant to which Merger Sub will be merged with and into the Company, and the Company will become a wholly owned subsidiary of Parent (the “Merger”);

WHEREAS, as of the date hereof, each Stockholder is the Beneficial Owner (as defined below) of Subject Shares (as defined below); and

WHEREAS, in order to induce Parent and Merger Sub to enter into the Merger Agreement, each Stockholder has agreed to enter into this Agreement.

NOW, THEREFORE, in consideration of the foregoing premises and of the covenants and agreements set forth herein and in the Merger Agreement, and intending to be legally bound hereby, the parties agree as follows:

(a) “Beneficially Own” or “Beneficial Owner” with respect to any securities means having “beneficial ownership” as determined pursuant to Rule 13d-3 under the Securities Exchange Act of 1934, as amended (the “Exchange Act”).

(b) “Company Capital Stock” means shares of common stock, par value $0.001 per share, of the Company and preferred stock, par value $0.001 per share, of the Company.

(c) “Company Options and Other Rights” means options, warrants and other rights to acquire, directly or indirectly, shares of Company Capital Stock.

(d) “Expiration Date” means the earlier to occur of (i) the Effective Time (as defined in the Merger Agreement) or (ii) the date on which the Merger Agreement is terminated pursuant to its terms.

(e) “Subject Shares” means, with respect to each Stockholder, (i) all shares of Company Capital Stock Beneficially Owned by such Stockholder as of the date of this Agreement; and (ii) all additional shares of Company Capital Stock of which such Stockholder acquires Beneficial Ownership during the period from the date of this Agreement through the Expiration Date.

(a) Each Stockholder hereby agrees that, prior to the Expiration Date, at any meeting of the stockholders of the Company, however called and at every adjournment and postponement thereof, and in any written action by consent of stockholders of the Company, unless otherwise directed in writing by Parent, such Stockholder shall cause to be counted as present thereat for purposes of establishing a quorum and shall vote, or cause to be voted, any and all Subject Shares as of the record date of such meeting or written consent:

(i) in favor of the Merger, the execution and delivery by the Company of the Merger Agreement and the adoption and approval of the Merger Agreement and the terms thereof, in favor of each of the other actions contemplated by the Merger Agreement and in favor of any action in furtherance of any of the foregoing;

(ii) against any action or agreement that would result or could reasonably be expected to result in a breach of any representation, warranty, covenant or obligation of the Company in the Merger Agreement; and

(iii) against the following actions (other than the Merger and the transactions contemplated by the Merger Agreement): (A) any extraordinary corporate transaction, such as a merger, consolidation or other business combination involving the Company or any subsidiary of the Company; (B) any sale, lease, sublease, license, sublicense or transfer of a material portion of the rights or other assets of the Company or any subsidiary of the Company; (C) any reorganization, recapitalization, dissolution or liquidation of the Company or any subsidiary of the Company; (D) any change in the individuals who serve as members of the board of directors of the Company; (E) any amendment to the Company’s certificate of incorporation or bylaws; (F) any material change in the capitalization of the Company or the Company’s corporate structure; and (G) any other action which is intended, or could reasonably be expected, to impede, interfere with, delay, postpone, discourage or adversely affect the Merger or any of the other transactions contemplated by the Merger Agreement or this Agreement.

(b) Prior to the Expiration Date, no Stockholder shall enter into any agreement or understanding with any Person to vote or give instructions in any manner inconsistent with clause “(i),” clause “(ii)” or clause “(iii)” of this Section 2(a).

(c) Each Stockholder hereby waives and agrees not to exercise any applicable “appraisal rights” under the Delaware General Corporation Law with respect to the Subject Shares in connection with the Merger and the Merger Agreement.

(a) In furtherance of the transactions contemplated hereby and by the Merger Agreement, and in order to secure the performance by each Stockholder of such Stockholder’s duties under this Agreement, such Stockholder, concurrently with the execution of this

2

Agreement, shall execute, in accordance with the provisions of applicable Delaware law, and deliver to Parent an irrevocable proxy, substantially in the form of Annex A hereto, and irrevocably appoint Parent or its designees, and each of them individually, with full power of substitution and resubstitution, such Stockholder’s attorney-in-fact and proxy to vote, or, if applicable, to give consent with respect to, all of the Subject Shares as of the record date of such vote or consent in respect of any of the matters set forth in, and in accordance with the provisions of, Section 2(a) (the “Proxy”).

(b) Each Stockholder understands and acknowledges that Parent is entering into the Merger Agreement in reliance upon such Proxy. Each Stockholder hereby affirms that the Proxy set forth in this Section 3 is given to secure the performance of the duties of such Stockholder under this Agreement. Each Stockholder hereby affirms that the irrevocable proxy is coupled with an interest and may under no circumstances be revoked prior to the Expiration Date. Each Stockholder hereby ratifies and confirms all that such irrevocable proxy may lawfully do or cause to be done by virtue hereof.

(c) With respect to the Stockholders, the Stockholders and the Company hereby waive, solely with respect to this Agreement, Section 4.1 of the Company’s 2009 Stockholders Agreement (the “Stockholders’ Agreement”), which Stockholders Agreement shall be terminated in connection with the consummation of the Merger and each Stockholder hereby revokes any and all prior proxies or powers of attorney given by such Stockholder prior to the date of this Agreement with respect to the voting of the Subject Shares and agrees not to grant any subsequent proxies or powers of attorney on or following the date of this Agreement with respect to the voting of the Subject Shares until the Expiration Date.

(d) Each Stockholder shall, at such Stockholder’s own expense, perform such further acts and execute such further proxies and other documents and instruments as may reasonably be required to vest in Parent the power to carry out and give effect to the provisions of this Agreement.

4. Covenants of Stockholders. Each Stockholder covenants and agrees for the benefit of Parent that, until the Expiration Date, such Stockholder will not:

(a) sell, transfer, pledge, hypothecate, encumber, assign, tender or otherwise dispose of, or enter into any contract, option or other arrangement or understanding (whether written or oral) with respect to the sale, transfer, pledge, hypothecation, encumbrance, assignment, tender or other disposition of, (i) any Subject Shares or any interest therein, or (ii) any Company Options and Other Rights or any interest therein; provided, however, that such Stockholder may convert, exercise or exchange Company Options and Other Rights into or for shares of Company Capital Stock in which event such shares of Capital Stock shall become and be deemed Subject Shares subject to all the terms and conditions of this Agreement;

(b) other than the Proxy, grant any powers of attorney or proxies or consents in respect of any of the Subject Shares, deposit any of such Subject Shares into a voting trust, or enter into a voting agreement with respect to any of such Subject Shares; and

3

(c) take any other action with respect to the Subject Shares that would in any way restrict, limit or interfere with the performance of such Stockholder’s obligations hereunder or the transactions contemplated hereby and the Merger Agreement.

5. Representations and Warranties of Stockholders. Each Stockholder represents and warrants to Parent as follows:

(a) As of the date of this Agreement and at all times through the Expiration Date:

(i) Such Stockholder is and will be the Beneficial Owner (free and clear of any encumbrances or restrictions) of the outstanding shares of Company Capital Stock set forth across from the heading “Shares of Company Capital Stock Beneficially Owned” below such Stockholders’ name on the signature page hereof.

(ii) Such Stockholder is and will be the Beneficial Owner (free and clear of any encumbrances or restrictions) of the outstanding Company Options and Other Rights set forth across from the heading “Company Options and Other Rights Beneficially Owned” below such Stockholders’ name on the signature page hereof (except to the extent that such Company Options and Other Rights are converted into, exercised or exchanged for shares of Company Capital Stock); and

(iii) Such Stockholder does not directly or indirectly Beneficially Own any shares of Company Capital Stock or Company Options or Other Rights or other securities of the Company, other than the shares of Company Capital Stock and Company Options and Other Rights set forth below such Stockholders’ name on the signature page hereof.

(b) Such Stockholder has and will have the legal capacity, power and authority to enter into and perform all of such Stockholder’s obligations under this Agreement and the Proxy. This Agreement has been duly executed and delivered by such Stockholder and, if such Stockholder is a corporation or partnership, has been duly authorized by all requisite corporate or partnership action of such Stockholder, as the case may be, and upon its execution and delivery by Parent, will constitute a legal, valid and binding obligation of such Stockholder, enforceable against such Stockholder in accordance with its terms, except as enforceability may be limited by bankruptcy, insolvency, reorganization, moratorium or other similar laws affecting or relating to creditors rights generally, and the availability of injunctive relief and other equitable remedies.

(c) The execution, delivery and performance by such Stockholder of this Agreement will not (i) conflict with, require a consent, waiver or approval under, or result in a breach of or default under, any of the terms of any contract, commitment or other obligation (written or oral) to which such Stockholder is a party or by which any of such Stockholder’s assets may be bound, and, if such Stockholder is a corporation or partnership, the organizational documents of such Stockholder, or (ii) violate any order, writ injunction, decree, judgment, order, statute, rule or regulation applicable to such Stockholder or any of its assets.

4

(d) No filing with, and no permit, authorization, consent or approval of, any state or federal public body or authority is necessary for the execution of this Agreement by such Stockholder and the consummation by such Stockholder of the transactions contemplated hereby.

6. Adjustments; Additional Shares. In the event (a) of any stock dividend, stock split, merger, recapitalization, reclassification, combination, exchange of shares or the like of the capital stock of the Company on, of or affecting the Subject Shares or (b) that a Stockholder shall become the Beneficial Owner of any additional shares of Company Capital Stock or other securities entitling the holder thereof to vote or give consent with respect to the matters set forth in Section 2(a), then the terms of this Agreement shall apply to the shares of Company Capital Stock or other instruments or documents held by such Stockholder immediately following the effectiveness of the events described in clause (a) or such Stockholder becoming the Beneficial Owner thereof as described in clause (b), as though, in either case, they were Subject Shares hereunder.

7. Amendments and Waivers. Any provision of this Agreement may be amended or waived if, and only if, such amendment or waiver is in writing and is signed, in the case of an amendment, by each party to this Agreement, or in the case of a waiver, by the party against whom the waiver is to be effective. No failure or delay by any party in exercising any right or privilege hereunder shall operate as a waiver thereof, nor shall any single or partial exercise thereof preclude any other or further exercise thereof or the exercise of any other right, power or privilege. To the maximum extent permitted by applicable law, (a) no waiver that may be given by a party shall be applicable except in the specific instance for which it was given and (b) no notice to or demand on one party shall be deemed to be a waiver of any obligation of such party or the right of the party giving such notice or demand to take further action without notice or demand.

8. Assignment. Neither this Agreement nor any of the rights, interests or obligations hereunder shall be assigned by either party hereto (whether by operation of law or otherwise) without the prior written consent of the other party. This Agreement shall be binding upon and shall inure to the benefit of the parties hereto and their respective successors and assigns. Any purported assignment not permitted under this Section 8 shall be null and void.

9. Entire Agreement. This Agreement and the documents, instruments and other agreements specifically referred to herein or delivered pursuant hereto, set forth the entire understanding of the parties with respect to the subject matter hereof. Any and all previous agreements and understandings between or among the parties regarding the subject matter hereof, whether written or oral, are superseded by this Agreement.

10. Notices. Any notice, request, demand, waiver, consent, approval or other communication which is required or permitted hereunder shall be in writing and shall be deemed given (a) on the date established by the sender as having been delivered personally; (b) on the date delivered by a private courier as established by the sender by evidence obtained from the courier; (c) on the date sent by facsimile or electronic mail, with confirmation of transmission, if sent during normal business hours of the recipient, if not, then on the next business day; or (d) on the fifth day after the date mailed, by certified or registered mail, return receipt requested, postage prepaid. Such communications, to be valid, must be addressed as follows:

If to Parent or Merger Sub, to:

Chiesi Farmaceutici S.p.A.

Via Palermo 26/A

43122 Parma, Italy

Attn: Marco Vecchia

Facsimile: +39 0521 774468

5

With a required copy to:

Morgan, Lewis & Bockius LLP

101 Park Avenue

New York, NY 10178

Facsimile: (212) 309-6001

If to Company, to:

Cornerstone Therapeutics Inc.

1255 Crescent Green Drive, Suite 250

Cary, NC 27518

Attn: Craig A. Collard

Facsimile: (919) 678-6599

With a required copy to:

Clifford Chance LLP

51 West 52nd Street

New York, NY 10019

Attn: John Healy

Facsimile: (212) 878-8375

If to a Stockholder, to the address set forth below such Stockholder’s name on the signature page hereto.

or to such other address or to the attention of such person or persons as the recipient party has specified by prior written notice to the sending party (or in the case of counsel, to such other readily ascertainable business address as such counsel may hereafter maintain). If more than one method for sending notice as set forth above is used, the earliest notice date established as set forth above shall control.

11. Captions. All captions contained in this Agreement are for convenience of reference only, do not form a part of this Agreement and shall not affect in any way the meaning or interpretation of this Agreement.

6

12. Counterparts. This Agreement may be executed in two or more consecutive counterparts (including by facsimile, or “.pdf” transmission), each of which shall be deemed to be an original, with the same effect as if the signatures thereto and hereto were upon the same instrument, and shall become effective when one or more counterparts have been signed by each of the parties and delivered (electronically or otherwise) to the other parties.

13. Severability; Enforcement. Any term or provision of this Agreement which is invalid or unenforceable in any jurisdiction shall, as to that jurisdiction, be ineffective to the sole extent of such invalidity or unenforceability without rendering invalid or unenforceable the remainder of such term or provision or the remaining terms and provisions of this Agreement in any jurisdiction. If any provision of this Agreement is so broad as to be unenforceable, such provision shall be interpreted to be only so broad as is enforceable.

14. Specific Performance. Stockholder acknowledges that the agreements contained in this Agreement are an integral part of the transactions contemplated by the Merger Agreement, and that, without these agreements, Parent would not enter into the Merger Agreement, and acknowledges that damages would be an inadequate remedy for any breach by Stockholder of the provisions of this Agreement. Accordingly, Stockholder agrees that Stockholder’s obligations hereunder shall be specifically enforceable and Stockholder shall not take any action to impede the other from seeking to enforce such right of specific performance.

15. Consent to Jurisdiction. Each of the parties hereto irrevocably agrees that any legal action or proceeding with respect to this Agreement and the rights and obligations arising hereunder, or for recognition and enforcement of any judgment in respect of this Agreement and the rights and obligations arising hereunder brought by the other party hereto or its successors or assigns, shall be brought and determined exclusively in the Delaware Court of Chancery, or, if the Delaware Court of Chancery declines to accept jurisdiction over a particular matter, any federal court within the State of Delaware, or, if both the Delaware Court of Chancery and the federal courts within the State of Delaware decline to accept jurisdiction over a particular matter, any other state court within the State of Delaware, and, in each case, any appellate court therefrom. Each of the parties hereto hereby irrevocably submits with regard to any such action or proceeding for itself and in respect of its property, generally and unconditionally, to the personal jurisdiction of the aforesaid courts and agrees that it will not bring any action relating to this Agreement or any of the transactions contemplated by this Agreement in any court other than the aforesaid courts. Each of the parties hereto hereby irrevocably waives, and agrees not to assert as a defense, counterclaim or otherwise, in any action or proceeding with respect to this Agreement, (i) any claim that it is not personally subject to the jurisdiction of the above named courts for any reason other than the failure to serve in accordance with this Section 15, (ii) any claim that it or its property is exempt or immune from the jurisdiction of any such court or from any legal process commenced in such courts (whether through service of notice, attachment prior to judgment, attachment in aid of execution of judgment, execution of judgment or otherwise) and (iii) to the fullest extent permitted by the applicable law, any claim that (x) the suit, action or proceeding in such court is brought in an inconvenient forum, (y) the venue of such suit, action or proceeding is improper or (z) this Agreement, or the subject matter hereof, may not be enforced in or by such courts. Each of the parties hereto agrees that service of process upon such party in any such action or proceeding shall be effective if such process is given as a notice in accordance with Section 10. EACH OF THE PARTIES TO THIS AGREEMENT HEREBY

7

IRREVOCABLY WAIVES TO THE EXTENT PERMITTED BY APPLICABLE LAW ANY AND ALL RIGHT TO A TRIAL BY JURY IN ANY DIRECT OR INDIRECT ACTION, PROCEEDING OR COUNTERCLAIM ARISING OUT OF OR RELATING TO THIS AGREEMENT, THE TRANSACTIONS CONTEMPLATED HEREIN. EACH PARTY (A) CERTIFIES THAT NO REPRESENTATIVE, AGENT OR ATTORNEY OF ANY OTHER PARTY HAS REPRESENTED, EXPRESSLY OR OTHERWISE, THAT SUCH OTHER PARTY WOULD NOT, IN THE EVENT OF LITIGATION, SEEK TO ENFORCE THE FOREGOING WAIVER, (B) MAKES THIS WAIVER VOLUNTARILY, AND (C) ACKNOWLEDGES THAT EACH PARTY HAS BEEN INDUCED TO ENTER INTO THIS AGREEMENT BY, AMONG OTHER THINGS, THE MUTUAL WAIVERS CONTAINED IN THIS SECTION 15.

16. Governing Law. This Agreement and all claims or causes of action (whether in tort, contract or otherwise) that may be based upon, arise out of or relate to this Agreement or the negotiation, execution or performance of this Agreement (including any claim or cause of action based upon, arising out of or related to any representation or warranty made in or in connection with this Agreement) shall be governed by and construed in accordance with the laws of the State of Delaware, without giving effect to any choice or conflict of law provision or rule (whether of the State of Delaware or any other jurisdiction) that would cause the application of the laws of any jurisdiction other than the State of Delaware.

[REMAINDER OF PAGE INTENTIONALLY LEFT BLANK]

8

IN WITNESS WHEREOF, this Agreement has been duly executed by the parties hereto all as of the day and year first above written.

[Signature Page to Voting Agreement]

[Signature Page to Voting Agreement]

ANNEX A

IRREVOCABLE PROXY

Capitalized terms used but not defined herein shall have the meaning ascribed to such terms in the Voting Agreement, dated as of September 15, 2013, among Chiesi Farmaceutici S.p.A., an Italian company (“Parent”), Chiesi U.S. Corporation, a Delaware corporation and wholly owned subsidiary of Parent (“Merger Sub”), Cornerstone Therapeutics Inc, a Delaware corporation (the “Company”) and each of the stockholders of the Company signatory thereto (each a “Stockholder” and together, the “Stockholders”) (the “Voting Agreement”). A copy of the Voting Agreement is attached hereto and is incorporated by reference herein.

This Proxy is given to secure the performance of the duties of the undersigned Stockholder pursuant to the Voting Agreement and is granted in consideration of Parent entering into the Merger Agreement.

The undersigned Stockholder hereby irrevocably appoints Parent and Merger Sub, and each of them individually, the sole and exclusive attorneys-in-fact, agents and proxies, with full power of substitution and resubstitution in each of them, for the undersigned Stockholder and in the name, place and stead of the undersigned Stockholder, to vote or, if applicable, to give written consent, with respect to, all Subject Shares and which the undersigned Stockholder is or may be entitled to vote at any meeting of the Company held after the date hereof, whether annual or special and whether or not an adjourned meeting, or, if applicable, to give written consent with respect thereto, in accordance with the provisions of Section 2(a) of the Voting Agreement as follows:

(i) in favor of the Merger, the execution and delivery by the Company of the Merger Agreement and the adoption and approval of the Merger Agreement and the terms thereof, in favor of each of the other actions contemplated by the Merger Agreement and in favor of any action in furtherance of any of the foregoing;

(ii) against any action or agreement that would result in a breach of any representation, warranty, covenant or obligation of the Company in the Merger Agreement; and

(iii) against the following actions (other than the Merger and the transactions contemplated by the Merger Agreement): (A) any extraordinary corporate transaction, such as a merger, consolidation or other business combination involving the Company or any subsidiary of the Company; (B) any sale, lease, sublease, license, sublicense or transfer of a material portion of the rights or other assets of the Company or any subsidiary of the Company; (C) any reorganization, recapitalization, dissolution or liquidation of the Company or any subsidiary of the Company; (D) any change in the individuals who serve as members of the board of directors of the Company; (E) any amendment to the Company’s certificate of incorporation or bylaws; (F) any material change in the capitalization of the Company or the Company’s corporate structure; and (G) any other action which is intended, or could reasonably be expected, to impede, interfere with, delay, postpone, discourage or adversely affect the Merger or any of the other transactions contemplated by the Merger Agreement or this Agreement.

This Proxy is coupled with an interest, shall be irrevocable to the fullest extent permitted by law and shall be binding on any successor in interest of the undersigned Stockholder. This Proxy shall not be terminated by operation of law upon the occurrence of any event, including, without limitation, the death or incapacity of the undersigned Stockholder.

This Proxy shall operate to revoke any prior proxy as to the Subject Shares heretofore granted by the undersigned Stockholder with respect to the subject matter of the Voting Agreement and the Merger Agreement.

This Proxy shall terminate on the Expiration Date.

SIGNATURE TO IRREVOCABLE PROXY

ANNEX A

IRREVOCABLE PROXY

Capitalized terms used but not defined herein shall have the meaning ascribed to such terms in the Voting Agreement, dated as of September 15, 2013, among Chiesi Farmaceutici S.p.A., an Italian company (“Parent”), Chiesi U.S. Corporation, a Delaware corporation and wholly owned subsidiary of Parent (“Merger Sub”), Cornerstone Therapeutics Inc, a Delaware corporation (the “Company”) and each of the stockholders of the Company signatory thereto (each a “Stockholder” and together, the “Stockholders”) (the “Voting Agreement”). A copy of the Voting Agreement is attached hereto and is incorporated by reference herein.

This Proxy is given to secure the performance of the duties of the undersigned Stockholder pursuant to the Voting Agreement and is granted in consideration of Parent entering into the Merger Agreement.

The undersigned Stockholder hereby irrevocably appoints Parent and Merger Sub, and each of them individually, the sole and exclusive attorneys-in-fact, agents and proxies, with full power of substitution and resubstitution in each of them, for the undersigned Stockholder and in the name, place and stead of the undersigned Stockholder, to vote or, if applicable, to give written consent, with respect to, all Subject Shares and which the undersigned Stockholder is or may be entitled to vote at any meeting of the Company held after the date hereof, whether annual or special and whether or not an adjourned meeting, or, if applicable, to give written consent with respect thereto, in accordance with the provisions of Section 2(a) of the Voting Agreement as follows:

(i) in favor of the Merger, the execution and delivery by the Company of the Merger Agreement and the adoption and approval of the Merger Agreement and the terms thereof, in favor of each of the other actions contemplated by the Merger Agreement and in favor of any action in furtherance of any of the foregoing;

(ii) against any action or agreement that would result in a breach of any representation, warranty, covenant or obligation of the Company in the Merger Agreement; and

(iii) against the following actions (other than the Merger and the transactions contemplated by the Merger Agreement): (A) any extraordinary corporate transaction, such as a merger, consolidation or other business combination involving the Company or any subsidiary of the Company; (B) any sale, lease, sublease, license, sublicense or transfer of a material portion of the rights or other assets of the Company or any subsidiary of the Company; (C) any reorganization, recapitalization, dissolution or liquidation of the Company or any subsidiary of the Company; (D) any change in the individuals who serve as members of the board of directors of the Company; (E) any amendment to the Company’s certificate of incorporation or bylaws; (F) any material change in the capitalization of the Company or the Company’s corporate structure; and (G) any other action which is intended, or could reasonably be expected, to impede, interfere with, delay, postpone, discourage or adversely affect the Merger or any of the other transactions contemplated by the Merger Agreement or this Agreement.

This Proxy is coupled with an interest, shall be irrevocable to the fullest extent permitted by law and shall be binding on any successor in interest of the undersigned Stockholder. This Proxy shall not be terminated by operation of law upon the occurrence of any event, including, without limitation, the death or incapacity of the undersigned Stockholder.

This Proxy shall operate to revoke any prior proxy as to the Subject Shares heretofore granted by the undersigned Stockholder with respect to the subject matter of the Voting Agreement and the Merger Agreement.

This Proxy shall terminate on the Expiration Date.

SIGNATURE TO IRREVOCABLE PROXY

EXHIBIT 10.18

JOINT FILING AGREEMENT

Each of the undersigned hereby agrees and consents that the Schedule 13D/A filed herewith (the “Schedule 13D/A”) by Cornerstone Biopharma Holdings, Ltd. is filed on behalf of each of them pursuant to the authorization of the undersigned to make such filing and that such Schedule 13D/A is filed jointly on behalf of each of them, pursuant to Sections 13(d) and 13(g) of the Securities Exchange Act of 1934, as amended, and the rules promulgated thereunder, including Rule 13d-1(k)(1). Each of the undersigned hereby agrees that such Schedule 13D/A is, and any further amendments to the Schedule 13D/A will be, filed on behalf of each of the undersigned. Each of the persons is not responsible for the completeness or accuracy of the information concerning the other persons making this filing unless such person knows or has reason to believe that such information is inaccurate. This agreement may be signed in counterparts. This agreement is effective as of September 20, 2013.