X0306 4 2015-12-16 0 0001297184 Amphastar Pharmaceuticals, Inc. AMPH 0001033511 ZASLOFF MICHAEL A C/O AMPHASTAR PHARMACEUTICALS, INC. 11570 6TH STREET RANCHO CUCAMONGA CA 91730 1 0 0 0 Common Stock 2015-12-16 4 M 0 16538 11.68 A 42863 D Common Stock 2015-12-16 4 S 0 16538 14.194 D 26325 D Common Stock 2015-12-17 4 M 0 14232 11.68 A 40557 D Common Stock 2015-12-17 4 S 0 14232 14.2013 D 26325 D Stock Option (right to buy) 11.68 2015-12-16 4 M 0 16538 0.00 D 2015-12-18 Common Stock 16538 14232 D Stock Option (right to buy) 11.68 2015-12-17 4 M 0 14232 0.00 D 2015-12-18 Common Stock 14232 0 D The sale reported in this Form 4 was effected pursuant to a Rule 10b5-1 trading plan adopted by the Reporting Person on June 10, 2015. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $14.03 to $14.37, 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 pricewithin the ranges set forth in footnotes (2) and (3) to this Form 4. The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $14.02 to $14.39, inclusive. Shares subject to the option are fully vested and immediately exercisable. /s/ Ken Stupak, by power of attorney 2015-12-17