Drug allergies are a group of symptoms caused by allergic reaction to a drug (medication).

Adverse reactions to drugs are common, and almost any drug can cause an adverse reaction. Reactions range from irritating or mild side effects such as nausea and vomiting to life-threatening anaphylaxis.

A true drug allergy results from a series of chemical steps within the body that produce the allergic reaction to a medication. One time (often the first time you take the drug), your immune system launches an incorrect response that is not noticeable. The next time you take the drug, an immune response occurs, and your body produces antibodies and histamine.

Most drug allergies cause minor skin rashes and hives. Serum sickness is a delayed type of drug allergy that occurs a week or more after exposure to a medication or vaccine.

Penicillin and related antibiotics are the most common cause of drug allergies. Other common allergy-causing drugs include:

• Sulfa drugs
• Anticonvulsants
• Insulin preparations (particularly animal sources of insulin)
• Iodinated (containing iodine) x-ray contrast dyes (these can cause allergy-like anaphylactoid reactions)

Most side effects of drugs are not due to an allergic reaction. For example, aspirin can cause nonallergic hives or trigger asthma. Some drug reactions are considered "idiosyncratic." This means the reaction is an unusual effect of the medication, not due to a predictable chemical effect of the drug. Many people confuse an uncomfortable, but not serious, side effect of a medicine (such as nausea) with a true drug allergy, which can be life threatening.

• Anaphylaxis, or severe allergic reaction (see below)
• Hives (a less common type of rash)
• Itching of the skin or eyes (common)
• Skin rash (common)
• Swelling of the lips, tongue, or face
• Wheezing

Symptoms of anaphylaxis include:

• Abdominal pain or cramping
• Confusion
• Diarrhea
• Difficulty breathing with wheeze or hoarse voice
• Dizziness
• Fainting, light-headedness
• Hives over different parts of the body
• Nausea, vomiting
• Rapid pulse
• Sensation of feeling the heart beat (palpitations)

An examination of the skin and face may show hives, rash, or angioedema (swelling of the lips, face, or tongue). Decreased blood pressure, wheezing, and other signs may indicate an anaphylactic reaction.

Skin testing may confirm allergy to penicillin-type medications. Testing may be ineffective (or in some cases, dangerous) for other medications. A history of allergic-type reaction after use of a medication is often considered proof enough of drug allergy -- no further testing is required. The same applies to other substances that are not considered drugs but are used in hospitals, such as x-ray contrast dyes.

The treatment goal is to relieve symptoms and prevent a severe reaction.

Treatment may include:

• Antihistamines to relieve mild symptoms such as rash, hives, and itching
• Bronchodilators such as albuterol to reduce asthma-like symptoms (moderate wheezing or cough)
• Corticosteroids applied to the skin, given by mouth, or given intravenously (directly into a vein)
• Epinephrine by injection to treat anaphylaxis

The offending medication and similar drugs should be avoided. Make sure all your health care providers -- including dentists and hospital personnel -- know about any drug allergies that you or your children have.

Identifying jewelry or cards (such as Medic-Alert or others) may be recommended.

Occasionally, a penicillin (or other drug) allergy responds to desensitization, where increasing doses of a medicine are given to improve a person's tolerance of the drug. This should only be done by an allergist.

Most drug allergies respond readily to treatment. A few cases cause severe asthma, anaphylaxis, or death.

• Anaphylaxis (life-threatening)
• Asthma
• Death

Call your health care provider if you are taking a medication and seem to be having a reaction to it.

Go to the emergency room or call the local emergency number (such as 911) if you have difficulty breathing or develop other symptoms of severe asthma or anaphylaxis. These are emergency conditions.

There is generally no way to prevent development of a drug allergy.

If you have a known drug allergy, avoiding the medication is the best way to prevent an allergic reaction. You may also be told to avoid similar medicines. For example, if you are allergic to penicillin, you should also avoid amoxicillin or ampicillin.

In some cases, a doctor may approve use of a drug that causes an allergy if you are pre-treated with corticosteroids (such as prednisone) and antihistamines (such as diphenhydramine). Do not try this without a doctor's supervision. Pre-treatment with corticosteroids and antihistamines has been shown to prevent anaphylaxis in people needing to get iodinated x-ray contrast dye.

Adkinson NF Jr. Drug Allergy. In Adkinson NF Jr., Yunginger JW, Busse WW, Bochner BS, Holgate ST, eds. Middleton's Allergy: Principles and Practice. 6th ed. Philadelphia, Pa; Mosby Elsevier; 2003: chap 92.

Grammer LC. Drug Allergy. In: Goldman L, Ausiello D, eds. Cecil Medicine. 23rd ed. Philadelphia, Pa: Saunders Elsevier; 2007: chap 275.