Penicillin allergy is the most common form of drug allergy. It is a serious condition whose symptoms may range from developing rashes, itchiness and swelling of the tongue, lips or the face. In severe cases, it may even be life-thretening as very sensitive indiduals may experience anaphylactic shock.

Penicillin and other related antibiotics falling under the so-called beta-lactam family of antibiotics are commonly prescribed because they are generally effective for common infections, and are relatively cheap. So if you are diagnosed to be highly sensitive to penicillin (may be determined using a skin test), it is vital that you let your doctor know of your allergies. It might also be wise to carry Penicillin Allergy Medical ID Tags which might save your life in cases of emergency.

The Mayo Clinic website has an excellent section on penicillin allergies.