Penicillin is a family of antibiotics used for fighting many bacterial infections. According to Mayo Clinic, medications in the penicillin group work both by killing target bacteria and by inhibiting their growth and reproduction inside the body.
According to Mayo Clinic, penicillins such as Amoxil and Pfizerpen are approved for treating a wide variety of bacterial infections, but drugs in this class generally cannot be substituted with other drugs in the same class. None of the penicillins are effective against viral infections because antibiotics are generally unable to disrupt viral reproduction. Drugs in the penicillin group may also be prescribed "off-label" by a doctor to treat ailments that are not officially recognized as treatable by such medications. Amoxicillin and ampicillin, for example, are not approved as treatments for chlamydia in pregnant women, but in such cases, they may be prescribed on an experimental basis. Experimental use of penicillin is also common for treating gastric ulcers, Lyme disease and typhoid fever. Penicillin-group drugs can be administered orally, in pill form or suspension or by injection. The exact drug, as well as its dose and method of delivery, is usually determined by the severity and location in the body of the infection.