X0306 4 2016-05-17 0 0001297184 Amphastar Pharmaceuticals, Inc. AMPH 0001047579 PRINS RICHARD K C/O AMPHASTAR PHARMACEUTICALS, INC. 11570 6TH STREET RANCHO CUCAMONGA CA 91730 1 0 0 0 Common Stock 2016-05-17 4 S 0 3200 13.5598 D 23000 D The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $13.50 to $13.66, 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. /s/ Ken Stupak, by power of attorney 2016-05-19