Although penicillin does not necessarily go bad, it should not be taken past its expiry date as it will lose effectiveness with time, according to Mayo Clinic. When doctors prescribe penicillin, patients need to take the full course of the antibiotics to clear up the infection completely. If a dosage is missed, the patient should take it as soon as possible but should not double the dose.
According to Mayo Clinic, penicillin is most effective when taken with a full glass of water on an empty stomach, and it should generally be taken either 1 hour before or 2 hours after meals. Also, patients should take their doses at evenly spaced times for optimal effectiveness. They should also store the penicillin in closed containers at room temperature, away from excess heat and moisture and direct sunlight.