Causes

Penicillin allergy occurs when your immune system becomes hypersensitive to the drug — mistakenly reacting to the drug as a harmful substance, as if it were a viral or bacterial infection.

Before the immune system can become sensitive to penicillin, you have to be exposed to the medication at least once. If and when your immune system misidentifies penicillin as a harmful substance, it develops an antibody to the drug.

The next time you take the drug, these specific antibodies flag it and direct immune system attacks on the substance. Chemicals released by this activity cause the signs and symptoms associated with an allergic reaction.

Previous exposure to penicillin may not be obvious. Some evidence suggests that trace amounts of it in the food supply may be sufficient for a person's immune system to create an antibody to it.

Penicillins and related drugs

Penicillins belong to a class of antibacterial drugs called beta-lactam antibiotics. Although the mechanisms of the drugs vary, generally they fight infections by attacking the walls of bacterial cells. In addition to penicillins, other beta-lactams more commonly associated with allergic reactions are a group called cephalosporins.

If you've had an allergic reaction to one type of penicillin, you may be — but are not necessarily — allergic to other types of penicillin or to some cephalosporins.

Penicillins include:

• Amoxicillin
• Ampicillin
• Dicloxacillin
• Nafcillin
• Oxacillin
• Penicillin G
• Penicillin V
• Piperacillin
• Ticarcillin

Cephalosporins include:

• Cefaclor
• Cefadroxil
• Cefazolin
• Cefdinir
• Cefepime (Maxipine)
• Cefotetan
• Cefprozil
• Cefuroxime
• Cephalexin (Keflex)