Amphastar Pharmaceuticals (AMPH) S-8Registration of securities for employees

Exhibit 5.1

March 16, 2020

Amphastar Pharmaceuticals, Inc.

11570 6th Street

Rancho Cucamonga, California 91730

Re:Registration Statement on Form S-8

Ladies and Gentlemen:

We have examined the Registration Statement on Form S-8 (the “Registration Statement”) to be filed by Amphastar Pharmaceuticals, Inc., a Delaware corporation, with the Securities and Exchange Commission on or about the date hereof, in connection with the registration under the Securities Act of 1933, as amended, of 1,164,425 shares (the “Shares”)  of your common stock, par value $0.0001 per share (the “Common Stock”)  reserved for issuance pursuant to the 2015 Equity Incentive Plan (the “Plan”).  As your legal counsel, we have reviewed the actions proposed to be taken by you in connection with the issuance and sale of the Shares to be issued under the Plan.

It is our opinion that the Shares, when issued and sold in the manner referred to in the Plan and pursuant to the agreements which accompany the Plan, will be legally and validly issued, fully paid and nonassessable.

We consent to the use of this opinion as an exhibit to the Registration Statement, and further consent to the use of our name wherever appearing in the Registration Statement and any amendments thereto.