Allergic reaction is an uncommon and unwanted side effect of medication.

Several different types of allergic reactions to medications can occur. Reactions to drugs range from a mild localized rash to serious effects on vital systems. The body's response can affect many organ systems, but the skin is the organ most frequently involved.

It is important to recognize the symptoms of a drug allergy, because they can be life-threatening. Death from an allergic reaction to a medication is extremely rare, however.

An allergic reaction does not often happen the first time you take a medication. A reaction is much more likely to occur the next time you take that medication. If you have a reaction the first time, you probably were exposed to the medication before without being aware of it.

Not all adverse reactions to drugs are allergies. In fact, fewer than 10% of adverse drug reactions are allergic. Other causes of adverse reactions are interactions between two or more drugs, inability to break the drug down completely in the body (as occurs with liver or kidney damage), overdose, and irritating side effects such as nausea, vomiting, or diarrhea.

If you have experienced a nonallergic drug reaction, it is important to describe it as such to medical personnel not as an allergy. If you don't know just describe the reaction the best way that you can.

An allergic reaction is caused by the body's immune system overreacting to the drug, which is viewed as a chemical "invader," or antigen. This overreaction is often called a hypersensitivity reaction.

The body produces antibodies to the antigen and stores the antibodies on special cells. The antibody in an allergic reaction is called immunoglobulin E, or IgE.

When the body is exposed to the drug again, the antibodies signal the cells to release chemicals called "mediators." Histamine is an example of a mediator.

The effects of these mediators on organs and other cells cause the symptoms of the reaction. The most common triggers of drug allergies are the following.

Painkillers (called analgesics) such as codeine, morphine, nonsteroidal antiinflammatory drugs (NSAIDs, such as ibuprofen or indomethacin), and aspirin. Antibiotics such as penicillin, sulfa drugs, and tetracycline. Antiseizure medications such as phenytoin (Dilantin) or carbamazepine (Tegretol).

Drug allergies may cause many different types of symptoms depending on the drug and the degree of exposure to the drug (how often you have taken it). These are the most common reactions: skin reactions, a measles-like rash, and hives. Slightly red, itchy, and raised swellings on the skin, which have an irregular shape. Photoallergy Sensitivity to sunlight, an itchy and scaly rash that occurs following sun exposure.

Erythema multiform Red, raised and itchy patches on the skin that sometimes look like bull's-eye targets and which may occur together with swelling of the face or tongue.