Penicillins are used to treat bacterial infections. There are many penicillin medications used to treat different kinds of infections. I am assuming you are referring to Penicillin G. This medication has been approved for use in nursing mothers by the American Academy of Pediatrics.

Penicillin medications generally enter human milk in small concentrations. It is still wise to watch your baby for diarrhea or an allergic response to this medication.

Reference: Medications and Mothers' Milk, Thomas Hale, R.Ph., Ph.D., 1997

Temporary weaning is rarely necessary when using a medication while breastfeeding, and could result in baby's refusal of the breast once you attempt to resume nursing. Most medications are compatible with nursing, but it is still very important to check with your Health Care Provider before using any medication while breastfeeding.