UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 20549

FORM 10-Q/A

Amendment No. 1

(Mark One)

For the quarterly period ended September 30, 2015

OR

For the transition period from                      to                     

RETROPHIN, INC.

(Exact name of registrant as specified in its charter)

12255 El Camino Real, Suite 250,

San Diego, CA 92130

(Address of Principal Executive Offices)

(760) 260-8600

(Registrant’s Telephone number including area code)

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Web site, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company.

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ¨    No  x

The number of shares of outstanding common stock, par value $0.0001 per share, of the Registrant as of November 2, 2015 was 36,148,930.

EXPLANATORY NOTE

This Amendment No. 1 to the Quarterly Report of Retrophin, Inc. on Form 10-Q/A (this “Form 10-Q/A”) amends our Quarterly Report on Form 10-Q for the period ended September 30, 2015, which was originally filed with the Securities and Exchange Commission (the “SEC”) on November 6, 2015 (the “Original Form 10-Q”). This Form 10-Q/A is being filed solely to refile Exhibit 10.1—Amendment No. 4 to Sublicense Agreement dated as of September 17, 2015, between Retrophin, Inc. and Ligand Pharmaceuticals Incorporated. In connection with the filing of this Form 10-Q/A and pursuant to the rules of the SEC, we are including with this Form 10-Q/A new certifications by our principal executive officer and principal financial officer.

Except as described above, no other changes have been made to the Original Form 10-Q, and this Form 10-Q/A does not amend, update or change the financial statements or disclosures in the Original Form 10-Q. This Form 10-Q/A does not reflect events occurring after the filing of the Original Form 10-Q or modify or update those disclosures. Accordingly, this Form 10-Q/A should be read in conjunction with our filings made with the SEC subsequent to the filing of the Original Form 10-Q, including any amendments to those filings.

PART II – Other information

Item 6. Exhibits

The exhibits listed in the Original Form 10-Q are required by Item 601 of Regulation S-K. A list of the exhibits filed with this Form 10-Q/A is provided below.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, as amended, the registrant caused this report to be signed on its behalf by the undersigned, thereunto duly authorized.