The Harvard Medical School Family Health Guide explains that the expiration date stamped on a medication does not indicate that the medication is unsafe. In fact, many medications are still safe to use even 10 years after expiration, producing no additional effects.
Penicillin is an antibiotic used to treat many common bacterial infections. According to the Mayo Clinic, some common side effects of penicillin include headache, diarrhea, vaginal discharge or white patches in the mouth. These side effects are likely to occur regardless of the expiration date of the medication. Rare side effects of taking penicillin include seizures, vomiting, depression and confusion. These side effects are not common. The Mayo Clinic advises those experiencing these symptoms to stop taking the medication and seek immediate medical assistance.
Penicillin was discovered in 1929 by Alexander Fleming. Because it was the first of its kind, it was touted as a miracle drug. Since that time, penicillin has been widely prescribed by health professionals to treat a variety of ailments. Because of the popularity of prescribing this medication, many people became resistant to the drug. Still others developed an allergy to penicillin. According to WebMD, penicillin allergies are among the most common drug allergies. Symptoms of an allergic reaction include a rash, hives, itchy eyes, and lip and tongue swelling.