Penicillin is an effective antibiotic that can prevent various bacterial infections, but the disadvantage is that overuse of penicillin may lead to a decreased effectiveness. This antibiotic works by stopping the growth of bacteria, according to WebMD.
Penicillin may not be safe for patients who have asthma, kidney disease, a blood clotting disorder, or another type of allergy. Penicillin does not treat viral infections such as flu or common cold. Diarrhea is a common side effect of penicillin. Another disadvantage of penicillin is that it may make oral birth control less effective, which may result in unplanned pregnancy.