Exhibit 5.1

June 25, 2014

Amphastar Pharmaceuticals, Inc.
11570 6th Street
Rancho Cucamonga, California 91730

Ladies and Gentlemen:

We have acted as counsel for Amphastar Pharmaceuticals, Inc., a Delaware corporation (the “Company”) in connection with the registration of shares of the Company’s common stock, $0.0001 par value per share (the “Common Stock”), pursuant to a Registration Statement on Form S-1 (File No. 333-196097) (as amended, the “Initial Registration Statement”) filed with the Securities and Exchange Commission (the “Commission”) under the Securities Act of 1933, as amended (the “Securities Act”), and a Registration Statement on Form S-1 relating to the Initial Registration Statement filed pursuant to Rule 462(b) promulgated under the Securities Act (the “462(b) Registration Statement” and, together with the Initial Registration Statement, the “Registration Statement”). The 462(b) Registration Statement relates to the registration of 736,000 shares of Common Stock (the “Additional Shares”).

You have requested our opinion as to the matters set forth below in connection with the Registration Statement. For purposes of rendering that opinion, we have examined: (i) the Registration Statement; (ii) the most recent prospectus included in the Registration Statement on file with the Commission as of the date of this opinion letter; (iii) the form of Amended and Restated Certificate of Incorporation of the Company and the form of Amended and Restated Bylaws of the Company, each of which have been filed with the Commission as an exhibit to the Registration Statement; and (iv) the records of corporate actions of the Company relating to the Registration Statement and the authorization for issuance and sale of the Shares, and matters in connection therewith. We have also made such other investigation as we have deemed appropriate. We have examined and relied upon certificates of public officials and, as to certain matters of fact that are material to our opinion, we have also relied on a certificate of an officer of the Company. In rendering our opinion, we also have made the assumptions that are customary in opinion letters of this kind. We have not verified any of those assumptions.

Our opinion set forth below is limited to the Delaware General Corporation Law, including the applicable provisions of the Delaware Constitution and reported judicial decisions interpreting those laws.

Based upon and subject to the foregoing, it is our opinion that the Additional Shares are duly authorized for issuance by the Company and, when issued and paid for in accordance with a definitive underwriting agreement approved by the Board of Directors of the Company, or a committee thereof, will be validly issued, fully paid and nonassessable.

We hereby consent to the filing of this opinion as an exhibit to the Registration Statement and to the reference to this firm in the related Prospectus under the caption “Legal Matters.” In giving our consent we do not hereby admit that we are in the category of persons whose consent is required under Section 7 of the Securities Act or the rules and regulations thereunder.