Penicillin is a group of antibiotics derived from Penicillium fungi, including penicillin G (IV use), penicillin V (oral use), procaine penicillin, and benzathine penicillin (intramuscular use).Penicillin antibiotics were among the first drugs to be effective against many previously serious diseases, such as syphilis and infections caused by staphylococci and streptococci. Penicillins are still widely used today, though many types of bacteria have now become resistant. All penicillins are β-lactam antibiotics and are used in the treatment of bacterial infections caused by susceptible, usually Gram-positive, organisms.