Penicillin was the first antibiotic introduced for clinical use. Sir Alexander Fleming used Penicillium molds to produce the original penicillin in the 1920s. Subsequent versions of penicillin were developed to decrease the adverse effects of the drug and to modify it to act on resistant bacteria. Penicillins include penicillin G benzathine (Bicillin, Permapen), penicillin G potassium (Pfizerpen), penicillin G procaine (Crysticillin-AS), penicillin V (Veetids), amoxicillin (Amoxil, Trimox), ampicillin (Principen), and ticarcillin (Ticar).
With the prolonged use of penicillin, more and more bacterial species have synthesized the enzyme penicillinase to counteract the effects of penicillin. Researchers have developed a group of drugs with a resistance to penicillinase, which allows them to remain effective against bacteria that are now resistant to the penicillins. Penicillin-resistant antibiotics include nafcillin and oxacillin. The actual drug chosen depends on the sensitivity of the bacteria causing the infection, the desired and available routes, and the personal experience of the clinician with the particular agent.