Filed by Xeris Pharmaceuticals, Inc.

pursuant to Rule 425 under the Securities Act of 1933

and deemed filed pursuant to Rule 14a-12

under the Securities Exchange Act of 1934

Subject Company: Strongbridge Biopharma plc

Commission File No.: 001-37569

Date: May 24, 2021

Xeris Pharmaceuticals, Inc. (“Xeris”) is using or making available the following communications through the website www.xerisstrongbridge.com (the “microsite”), a website maintained by Xeris providing information relating to the acquisition of Strongbridge Biopharma plc (“Strongbridge”):

Exhibit 1: Language from landing page the microsite entitled “Disclaimer Notice—Important”

Exhibit 2: Page of the microsite entitled “Homepage”

Exhibit 3: Page of the microsite entitled “Press Releases”

Exhibit 4: Page of the microsite entitled “Announcements”

Exhibit 5: Page of the microsite entitled “Filings”

Exhibit 6: Copy of the Xeris infographic under the “view infographic” button

Exhibit 1

Language from the landing page of the microsite entitled “Disclaimer Notice – Important”

Disclaimer Notice – Important

You are attempting to enter a website that is designated for the publication of documents and information in connection with the proposed offer (the Offer) by Xeris Pharmaceuticals, Inc. (Xeris) for Strongbridge Biopharma plc (Strongbridge).

The full terms and conditions of the Offer will be set out in the Joint Proxy Statement. In deciding whether or not to accept the Offer, shareholders of Strongbridge should rely only on the information contained and procedures described in the Joint Proxy Statement.

Terms defined in the announcement of the Offer dated 24 May 2021 shall have the same meaning when used in this notice.

ACCESS TO THIS WEBSITE MAY BE RESTRICTED UNDER SECURITIES LAWS IN CERTAIN JURISDICTIONS. THIS NOTICE REQUIRES YOU TO CONFIRM CERTAIN MATTERS (INCLUDING THAT YOU ARE NOT RESIDENT IN SUCH A JURISDICTION), BEFORE YOU MAY OBTAIN ACCESS TO THE INFORMATION. THE INFORMATION IS NOT DIRECTED AT, AND IS NOT INTENDED TO BE ACCESSIBLE BY, PERSONS RESIDENT IN ANY JURISDICTION WHERE TO DO SO WOULD CONSTITUTE A VIOLATION OF THE RELEVANT LAWS OF THAT JURISDICTION. IF YOU ARE NOT PERMITTED TO VIEW THE INFORMATION, OR VIEWING THE INFORMATION WOULD RESULT IN A BREACH OF THE ABOVE, OR YOU ARE IN ANY DOUBT AS TO WHETHER YOU ARE PERMITTED TO VIEW THE INFORMATION, PLEASE EXIT THIS WEBPAGE. THIS WEBSITE CONTAINS ANNOUNCEMENTS, DOCUMENTS AND INFORMATION (TOGETHER THE INFORMATION) RELATING TO THE OFFER IN COMPLIANCE WITH THE IRISH TAKEOVER PANEL ACT, 1997, TAKEOVER RULES 2013 (THE IRISH TAKEOVER RULES). THE INFORMATION IS BEING MADE AVAILABLE IN GOOD FAITH AND FOR INFORMATION PURPOSES ONLY, AND ITS AVAILABILITY IS SUBJECT TO THE TERMS AND CONDITIONS SET OUT BELOW. THE INFORMATION IS NOT INTENDED TO, AND DOES NOT, CONSTITUTE OR FORM ANY PART OF AN OFFER TO SELL OR OTHERWISE DISPOSE OF OR AN INVITATION OR THE SOLICITATION OF AN OFFER TO PURCHASE OR OTHERWISE ACQUIRE ANY SECURITIES, OR THE SOLICITATION OF A VOTE OR APPROVAL PURSUANT TO THE INFORMATION OR OTHERWISE. ANY PERSON SEEKING ACCESS TO THIS WEBSITE REPRESENTS AND WARRANTS TO XERIS AND STRONGBRIDGE THAT THEY ARE DOING SO FOR INFORMATION PURPOSES ONLY.

BASIS OF ACCESS TO INFORMATION

Please read this notice carefully before clicking “I agree” or “I disagree” below. This notice applies to all persons who view this website and, depending on where you live, it may affect your rights. This notice may be amended or updated by Xeris and Strongbridge from time to time and it should be read carefully in full each time you wish to view the website. In addition, the content of the website, and its accessibility by certain persons, may be amended at any time in whole or in part at the discretion of Xeris and Strongbridge. The Information speaks only at the date of the relevant document or announcement and Xeris and Strongbridge have, and accept, no responsibility or duty to update any Information (other than to the extent such duty arises as a matter of law or regulation).

The Xeris directors and the Xeris Biopharma Holdings, Inc. directors accept responsibility for the information contained in this website other than that relating to Strongbridge, the Strongbridge group, and the Strongbridge directors, and members of their immediate families, related trusts and persons connected with them, and for the statements made by Strongbridge in respect of Xeris and Xeris Biopharma Holdings, Inc. To the best of the knowledge and belief of the Xeris directors, the Xeris Biopharma Holdings, Inc. directors (who, in each case, have taken all reasonable care to ensure such is the case), the information contained in this website for which they respectively accept responsibility is in accordance with the facts and does not omit anything likely to affect the import of such information.

The Strongbridge directors accept responsibility for the information contained in this website relating to Strongbridge, the Strongbridge group and the Strongbridge directors and members of their immediate families, related trusts and persons connected with them, except for the statements made by Xeris in respect of Strongbridge. To the best of the knowledge and belief of the Strongbridge directors (who, in each case, have taken all reasonable care to ensure such is the case), the information contained in the materials on this website for which they respectively accept responsibility is in accordance with the facts and does not omit anything likely to affect the import of such information.

For regulatory reasons we have to ensure you are aware of the appropriate regulations for the country which you are in. To allow you to view details relating to the Offer, you have to read the following and then press “I agree”. If you are unable to agree you should press “I disagree” and you will not be able to view any such details.

OVERSEAS PERSONS

The Offer is being made for the securities of an Irish company and is subject to Irish disclosure requirements, which are different from those of other jurisdictions. As a consequence of legal restrictions, the release, publication or distribution of information contained on this website in certain jurisdictions or to certain persons may be restricted or unlawful. All persons who wish to view this website must first satisfy themselves that they are not subject to any local requirements in their jurisdiction of residence that prohibit or restrict them from doing so and should inform themselves of, and observe, any applicable legal or regulatory requirements applicable in their jurisdiction. If you are resident or located in a country that renders the accessing of this website or parts thereof illegal, whether or not subject to making certain notifications or taking other action, you should not view this website.

It is your responsibility to satisfy yourself as to the full observance of any relevant laws and regulatory requirements. If you are in any doubt, you should not continue to seek to access this website.

You should not forward, transmit or show the announcements, information or documents contained in this website to any person. In particular, you should not forward or transmit the announcements, information or documents contained therein to any jurisdiction where it would be unlawful to do so.

FORWARD-LOOKING STATEMENTS

The materials on this website, including information included or incorporated by reference, contain certain forward-looking statements with respect to a proposed transaction involving Xeris and Strongbridge and Xeris’, Strongbridge’s and/or the combined group’s estimated or anticipated future business, performance and results of operations and financial condition, including estimates, forecasts, targets and plans for Xeris and, following the acquisition, if completed, the combined group. The words “believe,” “expect,” “anticipate,” “project” and similar expressions, among others, generally identify forward-looking statements. These forward-looking statements are subject to risks and uncertainties that may cause actual results to differ materially from those indicated in the forward-looking statements. Such risks and uncertainties include, but are not limited to, the possibility that a possible acquisition will not be pursued, failure to obtain necessary shareholder or regulatory approvals or required financing or to satisfy any of the other conditions to the possible acquisition, the reaction of Xeris’ and Strongbridge’s shareholders to the proposed transaction, adverse effects on the market price of Xeris shares of common stock or Strongbridge ordinary shares and on Xeris’ or Strongbridge’s operating results because of a failure to complete the possible acquisition, failure to realize the expected benefits of the possible acquisition, failure to promptly and effectively integrate Strongbridge’s businesses, negative effects relating to the announcement of the possible acquisition or any further announcements relating to the possible acquisition or the consummation of the possible acquisition on the market price of Xeris shares of common stock or Strongbridge ordinary shares, significant transaction costs and/or unknown or inestimable liabilities, the risk that any potential payment of proceeds pursuant to the CVR Agreement may not be distributed at all or result in any value to Strongbridge shareholders, potential litigation associated with the possible acquisition, general economic and business conditions that affect the combined companies following the consummation of the possible acquisition, the impact of the COVID-19 pandemic on Xeris’ or Strongbridge’s businesses or the combined businesses following the consummation of the transaction, changes in global, political, economic, business, competitive, market and regulatory forces, future exchange and interest rates, changes in tax laws, regulations, rates and policies, future business acquisitions or disposals and competitive developments. These forward-looking statements are based on

numerous assumptions and assessments made in light of Xeris’ or, as the case may be, Strongbridge’s experience and perception of historical trends, current conditions, business strategies, operating environment, future developments and other factors it believes appropriate. By their nature, forward-looking statements involve known and unknown risks and uncertainties because they relate to events and depend on circumstances that will occur in the future. The factors described in the context of such forward-looking statements in these materials could cause Xeris’ plans with respect to Strongbridge, Strongbridge’s or Xeris’ actual results, performance or achievements, industry results and developments to differ materially from those expressed in or implied by such forward-looking statements. Although it is believed that the expectations reflected in such forward-looking statements are reasonable, no assurance can be given that such expectations will prove to have been correct and persons reading this notice are therefore cautioned not to place undue reliance on these forward-looking statements which speak only as the date thereof. Additional information about economic, competitive, governmental, technological and other factors that may affect Xeris is set forth in Item 1A, “Risk Factors,” in Xeris’ 2020 Annual Report on Form 10-K, which has been filed with the SEC. Additional information about economic, competitive, governmental, technological and other factors that may affect Strongbridge is set forth in Item 1A, “Risk Factors,” in Strongbridge’s 2020 Annual Report on Form 10-K, which has been filed with the SEC.

Any forward-looking statements in the materials on this website are based upon information available to Xeris, Strongbridge and/or their respective boards of directors, as the case may be, as of the date of the relevant material and, while believed to be true when made, may ultimately prove to be incorrect. Subject to any obligations under applicable law, none of Xeris, Strongbridge or any member of their respective boards of directors undertakes any obligation to update any forward-looking statement whether as a result of new information, future developments or otherwise, or to conform any forward-looking statement to actual results, future events, or to changes in expectations. All subsequent written and oral forward-looking statements attributable to Xeris, Strongbridge or their respective boards of directors or any person acting on behalf of any of them are expressly qualified in their entirety by this paragraph.

CONFIRMATION OF UNDERSTANDING AND ACCEPTANCE OF THIS NOTICE

By clicking on “I agree” below, you confirm that you have read, understood and agreed to be bound by the terms of the notice set out above and that you are not in, or a resident of, any jurisdiction where to download or view the Information would constitute a breach of securities law or regulation in that jurisdiction. If you click “I disagree” below, we will be unable to provide you with access to the Information and you will be redirected. Subject to any continuing obligations under applicable law or any relevant regulatory requirements, Xeris and Strongbridge expressly disclaim any obligation to disseminate, after the date of the posting of any document or announcement on this webpage, any updates or revisions to any statements in such documents or announcements in relation to the Offer to reflect any change in expectations or events, conditions or circumstances on which any such statements are based. If you are in any doubt about the contents of this website or the action you should take, you should seek your own financial advice from an appropriately authorized independent financial adviser.

Exhibit 2

Exhibit 3

Exhibit 4

Exhibit 5

Exhibit 6