Looked it up to be sure:
Over-the-counter topical acne medicines containing benzoyl peroxide or salicyclic acid are considered compatible with breastfeeding. Per Thomas Hale, PhD (author of Medications and Mothers' Milk), "The transcutaneous absorption of both of these drugs is minimal to nil. Peroxides, even if absorbed, would be instantly metabolized in the body and would not be able to reach the milk compartment."