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9 common allergies explained

By: Laura Moss on Jan. 11, 2011, 5:47 a.m.
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Penicillin is one of the most commonly prescribed antibiotics used to treat bacterial infections, and it's also one of the most common drug allergies. A penicillin allergy is an overreaction by your immune system to penicillin and related antibiotics.

Allergic reactions to penicillin can range from a rash to life-threatening anaphylaxis, which typically develops within an hour of taking the drug. Doctors aren't sure why some people develop a penicillin allergy, but once someone has had a reaction to it, the easiest way to prevent another reaction is to avoid penicillin and related antibiotics.