People sometimes mistake many adverse drug reactions for allergies. For example, people who experience stomach discomfort after taking aspirin (a common adverse reaction) often say they are "allergic" to aspirin. However, this is not a true allergic reaction. True allergic reactions involve activation of the immune system by the drug (see also Overview of Allergic Reactions). Aspirin use can cause stomach discomfort because aspirin interferes with the stomach's natural barrier defenses against stomach acid.
Allergic (usually hypersensitivity) reactions to a drug are relatively uncommon. In contrast to other types of adverse drug reactions, the number and severity of allergic reactions do not usually correlate with the amount of drug taken. For people who are allergic to a drug, even a small amount of the drug can trigger an allergic reaction (see Allergic Reactions: Introduction). These reactions range from minor and simply annoying to severe and life threatening. Examples are
Rashes and itching
Constriction of the airways and wheezing
Swelling of tissues (such as the voice box [larynx] and the opening between the vocal cords that closes to stop the flow of air to the lungs [glottis]), which impairs breathing
Fall in blood pressure, sometimes to dangerously low levels
Other types of allergic reactions to drugs are even less common than hypersensitivity reactions. They usually take days or weeks to develop and tend to last longer. These reactions involve different types of antibodies that react with a drug and attack different parts of the body. For example, they can affect red blood cells and lead to anemia or cause inflammation that can affect the skin, joints or kidneys.
Drug allergies cannot be anticipated, because reactions occur after a person has been previously exposed to the drug (whether it was applied to the skin, taken by mouth, or injected) one or more times without any allergic reaction. A mild reaction may be treated with an antihistamine. A severe or life-threatening anaphylactic reaction may require an injection of epinephrine (also called adrenaline) or a corticosteroid, such as hydrocortisone.
Before prescribing a new drug, doctors usually ask whether a person has any known drug allergies. People who have had severe allergic reactions should wear a medical alert necklace or bracelet inscribed with their drug allergies. This information (for example, penicillin allergy) can alert medical and paramedical personnel in case of an emergency.
The following are some English-language resources that may be useful. Please note that THE MANUAL is not responsible for the content of these resources.
Drugs Mentioned In This Article
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