Merck Manual

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Allergies to Drugs


Daphne E. Smith Marsh

, PharmD, BC-ADM, CDCES, University of Illinois at Chicago College of Pharmacy

Reviewed/Revised Mar 2023

People sometimes mistake many adverse drug reactions Overview of Adverse Drug Reactions Adverse drug reactions (adverse effects) are any unwanted effects of a drug or medication. In the early 1900s, German scientist Paul Ehrlich described an ideal drug as a "magic bullet." Such... read more 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 or medication (see also Overview of Allergic Reactions Overview of Allergic Reactions Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies cause sneezing, watery and itchy eyes, a... read more 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 drugs (including medications) 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 Overview of Allergic Reactions Allergic reactions (hypersensitivity reactions) are inappropriate responses of the immune system to a normally harmless substance. Usually, allergies cause sneezing, watery and itchy eyes, a... read more Overview of Allergic Reactions ). These reactions range from minor and simply annoying to severe and life threatening. Examples are

  • Rashes and itching

  • Fever

  • 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 Anaphylactic Reactions Anaphylactic reactions are sudden, widespread, potentially severe and life-threatening allergic reactions. Anaphylactic reactions often begin with a feeling of uneasiness, followed by tingling... read more may require an injection of epinephrine (also called adrenaline) or a corticosteroid, such as hydrocortisone.

Before prescribing a new medication, 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.

More Information

The following English-language resources may be useful. Please note that THE MANUAL is not responsible for the content of these resources.

Drugs Mentioned In This Article

Generic Name Select Brand Names
Anacin Adult Low Strength, Aspergum, Aspir-Low, Aspirtab , Aspir-Trin , Bayer Advanced Aspirin, Bayer Aspirin, Bayer Aspirin Extra Strength, Bayer Aspirin Plus, Bayer Aspirin Regimen, Bayer Children's Aspirin, Bayer Extra Strength, Bayer Extra Strength Plus, Bayer Genuine Aspirin, Bayer Low Dose Aspirin Regimen, Bayer Womens Aspirin , BeneHealth Aspirin, Bufferin, Bufferin Extra Strength, Bufferin Low Dose, DURLAZA, Easprin , Ecotrin, Ecotrin Low Strength, Genacote, Halfprin, MiniPrin, St. Joseph Adult Low Strength, St. Joseph Aspirin, VAZALORE, Zero Order Release Aspirin, ZORprin
Adrenaclick, Adrenalin, Auvi-Q, Epifrin, EpiPen, Epipen Jr , Primatene Mist, SYMJEPI, Twinject
A-Hydrocort, Ala-Cort, Ala-Scalp, Alkindi, Anucort-HC, Anumed-HC, Anusol HC, Aquaphor Children's Itch Relief, Aquaphor Itch Relief, Balneol for Her, Caldecort , Cetacort, Colocort , Cortaid, Cortaid Advanced, Cortaid Intensive Therapy, Cortaid Sensitive Skin, CortAlo, Cortef, Cortenema, Corticaine, Corticool, Cortifoam, Cortizone-10, Cortizone-10 Cooling Relief, Cortizone-10 External Itch Relief, Cortizone-10 Intensive Healing, Cortizone-10 Plus, Cortizone-10 Quick Shot, Cortizone-5 , Dermarest Dricort, Dermarest Eczema, Dermarest Itch Relief, Encort, First - Hydrocortisone, Gly-Cort , GRx HiCort, Hemmorex-HC, Hemorrhoidal-HC, Hemril , Hycort, Hydro Skin, Hydrocortisone in Absorbase, Hydrocortone, Hydroskin , Hydroxym, Hytone, Instacort, Lacticare HC, Locoid, Locoid Lipocream, MiCort-HC , Monistat Complete Care Instant Itch Relief Cream, Neosporin Eczema, NuCort , Nutracort, NuZon, Pandel, Penecort, Preparation H Hydrocortisone, Proctocort, Proctocream-HC, Procto-Kit, Procto-Med HC , Procto-Pak, Proctosert HC , Proctosol-HC, Proctozone-HC, Rectacort HC, Rectasol-HC, Rederm, Sarnol-HC, Scalacort, Scalpicin Anti-Itch, Solu-Cortef, Texacort, Tucks HC, Vagisil Anti-Itch, Walgreens Intensive Healing, Westcort
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