Penicillin antibiotics, which are available only by prescription, should not be used after the expiration date found on their prescription label, according to Mayo Clinic. One should follow all the prescribing doctor's orders in taking penicillins and dispose of any remaining medicine after a course of treatment.
Mayo Clinic cautions patients not to stop taking prescribed antibiotics as soon as they begin to feel better. Similarly, University Health Service of the University of Michigan says that patients should take all prescribed antibiotic medication as instructed by their doctor or pharmacist until it is gone and should take antibiotics only for the condition for which the doctor prescribed them. Following these instructions, a patient is unlikely to face the situation of taking possibly expired penicillin.