Exhibit 32.1

CERTIFICATIONS OF PRINCIPAL EXECUTIVE OFFICER

PURSUANT TO 18 U.S.C. SECTION 1350,

AS ADOPTED PURSUANT TO SECTION 906

OF THE SARBANES-OXLEY ACT OF 2002

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The undersigned officer of Amphastar Pharmaceuticals, Inc. (the “Company”), hereby certifies, to the best of such officer’s knowledge, that:

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(i) the Annual Report on Form 10-K of the Company for the year ended December 31, 2022 (the “Report”) fully complies with the requirements of Section 13(a) or Section 15(d) of the Securities Exchange Act of 1934 and

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(ii) the information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company at the dates and for the periods indicated.

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