X0306 4 2020-05-20 0 0001297184 Amphastar Pharmaceuticals, Inc. AMPH 0001379941 Lee Howard C/O AMPHASTAR PHARMACEUTICALS, INC. 11570 6TH STREET RANCHO CUCAMONGA CA 91730 1 0 0 0 Common Stock 2020-05-20 4 M 0 17782 15.11 A 145863 D Common Stock 2020-05-20 4 S 0 17782 18.4332 D 128081 D Common Stock 2020-05-21 4 M 0 10000 15.11 A 138081 D Common Stock 2020-05-21 4 S 0 10000 18.4687 D 128081 D Stock Option (right to buy) 15.11 2020-05-20 4 M 0 17782 0 D 2020-05-27 Common Stock 17782 30000 D Stock Option (right to buy) 15.11 2020-05-21 4 M 0 10000 0 D 2020-05-27 Common Stock 10000 20000 D The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $18.29 to $18.765, inclusive. The reporting person undertakes to provide to the Issuer, any security holder of the Issuer, or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares sold at each separate price within the ranges set forth in footnotes (1) and (2) to this Form 4. The price reported in Column 4 is weighted average price. These shares were sold in multiple transactions at prices ranging from $18.265 to $18.635, inclusive. Shares subject to the option are fully vested and immediately exercisable. /s/ Eva Wen, by power of attorney 2020-05-22