AMPH · Amphastar Pharmaceuticals, Inc.

Includes 910 shares acquired on May 31, 2026 under the Issuer's 2014 Employee Stock Purchase Plan.

The sale reported in this Form 4 was effected pursuant to a Rule 10b5-1 trading plan adopted by the reporting person on March 10, 2026.

Shares subject to the option are fully vested and immediately exercisable.

All of the shares subject to the option vest on July 9, 2027.

The reported shares are represented by restricted stock units, or RSUs, all of which vest on July 9, 2027.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of RSUs.

The reported shares are represented by restricted stock units, or RSUs, all of which vest on June 1, 2027.

All of the shares subject to the option vest on June 1, 2027.

The reported shares are represented by restricted stock units, or RSUs, all of which vest on June 1, 2027.

The reported shares are represented by restricted stock units, or RSUs, all of which vest on June 1, 2027.

All of the shares subject to the option vest on June 1, 2027.

The reported shares are represented by restricted stock units, or RSUs, all of which vest on June 1, 2027.

All of the shares subject to the option vest on June 1, 2027.

All of the shares subject to the option vest on June 1, 2027.

All of the shares subject to the option vest on June 1, 2027.

All of the shares subject to the option vest on June 1, 2027.

The reported shares are represented by restricted stock units, or RSUs, all of which vest on June 1, 2027.

The reported shares are represented by restricted stock units, or RSUs, all of which vest on June 1, 2027.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The securities are held of record by Mary Z. Luo. The reporting persons are husband and wife.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of RSUs.

The securities are held of record by Mary Z. Luo. The reporting persons are husband and wife.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of RSUs.

The shares subject to the option vest in four equal annual installments beginning on March 9, 2027.

The securities are held of record by Mary Z. Luo. The reporting persons are husband and wife.

The shares subject to the option vest in four equal annual installments beginning on March 9, 2027.

The reported shares are represented by restricted stock units, or RSUs, which vest in four equal annual installments beginning on March 9, 2027.

The shares subject to the option vest in four equal annual installments beginning on March 9, 2027.

The reported shares are represented by restricted stock units, or RSUs, which vest in four equal annual installments beginning on March 9, 2027.

The securities are held of record by Mary Z. Luo. The reporting persons are husband and wife.

The reported shares are represented by restricted stock units, or RSUs, which vest in four equal annual installments beginning on March 9, 2027.

The reported shares are represented by restricted stock units, or RSUs, which vest in four equal annual installments beginning on March 9, 2027.

The shares subject to the option vest in four equal annual installments beginning on March 9, 2027.

The reported shares are represented by restricted stock units, or RSUs, which vest in four equal annual installments beginning on March 9, 2027.

The shares subject to the option vest in four equal annual installments beginning on March 9, 2027.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The securities are held of record by Mary Z. Luo. The reporting persons are husband and wife.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

Includes 545 shares acquired on May 31, 2025 under the Issuer's 2014 Employee Stock Purchase Plan.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The securities are held of record by Mary Z. Luo. The reporting persons are husband and wife.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

The reported shares were withheld to satisfy the reporting person's tax liability in connection with the vesting of restricted stock units, or RSUs.

Includes 545 shares acquired on May 31, 2025 under the Issuer's 2014 Employee Stock Purchase Plan.

The price reported in Column 4 is a weighted average price. These shares were sold in multiple transactions at prices ranging from $25.97 to $26.50, 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.