Penicillin is in the pregnancy category B. This drug has shown no negative impact on pregnancy in women free of penicillin allergy. Diarrhea is often associated with antibiotic use. If diarrhea appears to contain blood or lasts for more than a few days, contact your physician as this could be a sign of an allergy to penicillin. Penicillin passes into breast milk in small amounts. There are no reported negative side effects associated with penicillin use while breastfeeding.