Penicillin is a group of antibiotics that includes penicillin G, penicillin V, procaine penicillin, and benzathine penicillin. Penicillin G, also known as benzylpenicillin, is administered intravenously or intramuscularly as it is unstable in the acidic stomach. Penicillin V can be administered orally as it is more acid stable. Benzylpenicillin is typically given via injection because it is unstable in the highly acidic stomach.