Facts About Drug Allergies

Finding out that you have an allergy to a drug can throw you. It can also bring up a lot of questions.

The eight facts below are a good start. Talk with your doctor if you’re still not sure about things after you read them.

  1. Anybody can be allergic to any drug. That includes prescription medications and ones you can get over the counter. Some drugs are much more likely to trigger allergic reactions, like some antibiotics, seizure medications, and some pain relievers.
  2. Symptoms can start right away or weeks later. You may start to feel things within minutes -- or even seconds -- of taking a drug. Others times, it may take days or weeks. That can make it harder for you and your doctor to figure out what caused the problem.
  3. Symptoms can be mild or severe. Drug allergies are most likely to affect your skin, causing rash or hives. They can also affect your nose, airways, ears, or lining of your stomach. For some people, they may be life-threatening.
  4. A drug allergy isn’t the same as a drug side effect. Having a bad side effect to a drug is common. It can be anything from a mild side effect (like upset stomach) to a serious problem (like damage to your heart). True drug allergies are different. When you have an allergy, your immune system is to blame. Your body mistakes something harmless -- like pollen, food, or a medicine -- for a hazard. Your immune system makes special cells and chemicals to fight it, which brings swelling and other symptoms.
  5. You can develop a drug allergy at any time. Even a drug you've been taking for years can trigger an allergic reaction.
  6. You may not have an allergic reaction the first time you take a drug. When you first take a drug, your body just becomes sensitized to it. The problem usually starts the next time you take it. By then, your body is prepared. As soon as it senses the drug in your body, it sounds the alarm and your immune system attacks.


People can have a reaction the first time they take a drug. It differs from person to person. Some people may have a reaction right away, while others might take the drug several times before a reaction occurs. Or, they may have taken the drug -- or a similar drug -- in the past and not know it. Sometimes, being exposed to a similar chemical in cosmetics is enough to sensitize a person.

  1. Some people have a greater risk of drug allergies. We don't know why people get drug allergies. Some things seem to increase the risk, such as:
  1. You can manage your drug allergy. It can be scary, and diagnosing it can be hard. But by working with your doctor, you can figure out your drug allergy and learn how to work around it.


WebMD Medical Reference Reviewed by Nayana Ambardekar, MD on June 13, 2020



American Academy of Allergy, Asthma, and Immunology: "Medications and Drug Allergic Reactions: Tips to Remember."

American College of Allergy, Asthma & Immunology: "Drug Reactions and Drug Allergies."

Levy, JH. Anaphylaxis and Adverse Drug Reactions, American Society of Anesthesiologists,  2005.

UpToDate: "An approach to the patient with drug allergy," "Rapid drug desensitization for immediate hypersensitivity reactions," "Drug Allergy: Classification and Clinical Features."

American Academy of Allergy Asthma and Immunology: “Drug Allergy Overview.”

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