What are side effects of penicillin?

many. This medication may cause stomach upset, diarrhea, nausea, and vomiting during the first few days as your body adjusts to the medication. If these symptoms persist or become severe, inform your doctor. Notify your doctor immediately if an allergic reaction occurs while taking this medication. Symptoms include: difficulty breathing, skin rash, hives, itching. If you notice other effects not listed.