Penicillin is a group of antibiotics, derived originally from common moulds known as Penicillium moulds; which includes penicillin G (intravenous use), penicillin V (use by mouth), procaine penicillin, and benzathine penicillin (intramuscular use). Penicillin antibiotics were among the first medications to be effective against many bacterial infections caused by staphylococci and streptococci. They are still widely used today, though many types of bacteria have developed resistance following extensive use.

About 10% of people report that they are allergic to penicillin; however, up to 90% of this group may not actually be allergic. Serious allergies only occur in about 0.03%.Those who are allergic to penicillin are most often given cephalosporin C because of its functional groups. All penicillins are β-lactam antibiotics, which are some of the most powerful and successful achievements in modern science.

Penicillin was discovered in 1928 by Scottish scientist Alexander Fleming. People began using it to treat infections in 1942. There are several enhanced penicillin families which are effective against additional bacteria; these include the antistaphylococcal penicillins, aminopenicillins and the antipseudomonal penicillins. They are derived from Penicillium fungi. Fleming shared the 1945 Nobel Prize in Physiology or Medicine for his discovery, along with Oxford University scientists Howard Florey and Ernst Boris Chain (who developed improved ways to produce and concentrate the drug and prove its antibacterial effects).