Health A-Z

Medical Content Created by the Faculty of the
Harvard Medical School

Medication Allergy

What Is It?

A true allergic reaction to medication occurs when the immune system is activated in response to a drug. The medication can be taken by mouth, injected into the body or rubbed on the skin. The symptoms from an allergic reaction vary from a mild skin rash to sudden swelling of many body parts with life threatening fall in blood pressure.

Most people with a drug allergy have been exposed to that drug or a similar drug before. During the earlier exposure, immune cells formed antibodies against the drug. Antibodies are proteins created by the immune system to battle foreign invaders such as bacteria and viruses. When a person is exposed to the drug again, the antibodies go into action, setting off the allergic response. The symptoms of drug allergy may happen immediately or after taking the drug for a week or more.

The reason a person develops a particular drug allergy is usually unknown, but genetics probably play a significant role.

Drug allergies can pose a significant problem, not only because of the symptoms they cause, but also because they can prevent or hinder the use of the more effective medications to treat medical conditions.

For many, medication allergies go undetected until they take a drug and have an allergic reaction.

Many people are sensitive to medications, but not all of these sensitivities are true allergic reactions. Some adverse reactions to drugs are side effects. Among the most common side effects are upset stomach, diarrhea, vomiting, fever and a skin reaction to sunlight called photosensitivity. However, drug allergies are not the same thing as side effects. Side effects do not involve the immune system, and sometimes can be avoided by lowering the dose. In order for the reaction to be an allergy, the immune system must be involved.