Does penicillin affect the birth control pill?

Answer 1

Study after study shows that there are no clinically significant drug interactions* between the vast majority of antibiotics and the Birth Control pill. Anecdotal evidence from health care providers has led some to recommend using a backup method of birth control for a week after using antibiotics. Long-term use (as for acne) does not increase the risk of pregnancy. There are a few antibiotics, such as rifampin and griseofulvin and some HIV medications, that do interact. The safest approach is to check with your health care provider or pharmacist for advice specific to your situation.

*While some antibiotics may slightly change the absorption of estrogen in the gut, these changes are not enough to change how well the birth control pill works, with the exceptions noted above.