Exhibit 5.1

May 24, 2013

Avanir Pharmaceuticals, Inc.

20 Enterprise, Suite 200

Aliso Viejo, CA 92656

Ladies and Gentlemen:

This opinion letter is furnished to you in connection with a registration statement on Form S-8 (the “Registration Statement”), filed with the Securities and Exchange Commission under the Securities Act of 1933, as amended, for the registration of 325,000 shares of Common Stock, $0.0001 par value (the “Shares”), of Avanir Pharmaceuticals, Inc., a Delaware corporation (the “Company”) that are issuable under the Company’s 2005 Equity Incentive Plan (the “2005 Plan”).

We are familiar with the actions taken by the Company in connection with the adoption of the 2005 Plan and the authorization of the Shares for issuance under the 2005 Plan. For purposes of our opinion, we have examined and relied upon such documents, records, certificates and other instruments as we have deemed necessary. The opinions expressed below are limited to the Delaware General Corporation Law, including the applicable provisions of the Delaware Constitution and the reported cases interpreting those laws.

For purposes of the opinion expressed below, we have assumed that a sufficient number of authorized but unissued shares of the Company’s Common Stock will be available for issuance when the Shares are issued.

Subject to the foregoing, we are of the opinion that the Shares have been duly authorized and, when the Shares have been issued and sold in accordance with the terms of the 2005 Plan, the Shares will be validly issued, fully paid and non-assessable.

We hereby consent to the filing of this opinion letter as an exhibit to the Registration Statement. Our consent shall not be deemed an admission that we are experts whose consent is required under Section 7 of the Securities Act of 1933 or the rules and regulations of the Securities and Exchange Commission thereunder.

It is understood that this opinion letter is to be used only in connection with the offer and sale of Shares while the Registration Statement is in effect.

Very truly yours,

/s/ Ropes & Gray LLP

Ropes & Gray LLP