Drug Allergies

A drug allergy is an allergic reaction to a normally harmless substance in a medication. Common causes include penicillin, sulfa drugs and NSAIDs. Symptoms can range from mild discomfort to life-threatening conditions. The best way to treat a drug allergy is to avoid it. You may need other medications to help treat drug allergy symptoms.


What are drug allergies?

Drug allergies are an allergic reaction to medications. It’s when your body’s immune system overreacts to medications. Your body develops a sensitivity to one of the substances in the medication and “remembers” it as a harmful foreign invader, like bacteria or viruses. The next time you ingest that drug, your immune system releases antibodies to remove it from your body.

A drug allergy can be deadly. If you or a loved one have severe drug allergy symptoms, including swelling of your tongue or throat, trouble breathing, a fast heartbeat, lightheadedness or unconsciousness, you must act quickly:

  • Call 911 or your local emergency number.
  • Go to your nearest emergency room (ER) as soon as possible.

What is an example of a drug allergy?

Any drug can cause an allergic reaction. But the most common medications that cause an allergic reaction include:

What is the most common drug allergy?

Penicillin is the most widely reported drug allergy. Around 10% of people report being allergic to penicillin, though recent studies show that you may outgrow penicillin allergies with time.

How common are drug allergies?

An adverse drug reaction is an unexpected or undesirable reaction to a medication. Drug allergies account for less than 10% of all adverse drug reactions. Side effects account for more than 90% of all adverse drug reactions. Side effects are possible reactions to a medicine that don’t involve your immune system.


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Symptoms and Causes

What are the signs and symptoms of a drug allergy?

An allergic reaction to medication usually begins within minutes after you take the drug. The symptoms and their severity can vary between people. Mild drug allergy symptoms may include:

Severe drug allergy symptoms may include anaphylaxis. Anaphylaxis is a serious allergic reaction that can be deadly without immediate treatment. Severe symptoms may include:

In rare instances, you may have an allergic reaction that slowly develops days or even weeks after taking a drug. The response can also last for days after you stop taking the medication. These reactions may include:

What causes drug allergies?

If you have a drug allergy, your immune system responds by making immunoglobulin E (IgE) after your first exposure to the drug (sensitization). IgE is a type of antibody that helps remove harmful substances from your body. Your body makes many different IgE types, which target specific types of allergens. The IgE travels to cells that contain histamine (mast cells) in your mucus membranes, skin, gastrointestinal (GI) tract and airways. The next time you encounter the drug allergen, the IgE attaches to the allergen and tells the mast cells to release histamine and other chemicals. Histamine causes your immediate allergy symptoms — they usually develop within minutes.

Your body’s T-cells may also cause an allergic reaction. T-cells are a type of white blood cell that helps protect your body from infection. Your T-cells recognize that the drug is a foreign body and bind to it. The immune response is slower and typically only affects your skin, like with hives, an itchy rash and so on.

How long does it take for a drug allergy to show up?

It depends. Some people are allergic to medicine the first time they ingest the drug. For others, they may not have a reaction until their second exposure or even after many exposures.

Are drug allergies contagious?

No, drug allergies aren’t contagious. You can’t spread a drug allergy to another person.

Who do drug allergies affect?

Anyone can have drug allergies. But you may be more likely to have or develop drug allergies if your biological parents have drug allergies.


Diagnosis and Tests

How is a drug allergy diagnosed?

An allergist can diagnose a drug allergy. An allergist is a healthcare provider who specializes in diagnosing through tests and determining the best possible treatment.

Before conducting drug allergy tests, they’ll review your medical history and may ask some questions, including:

  • When did you first notice symptoms?
  • What are your symptoms?
  • How long did your symptoms last?
  • Did you take anything to help relieve your symptoms?
  • Did you need to go to a doctor’s office or to the hospital to receive treatment for your symptoms?
  • Do you take vitamins or herbal supplements?
  • Do you have a family history of drug allergies?
  • What medications are you currently taking, including over-the-counter medications?
  • What medications have you taken over the last several weeks, including over-the-counter medications?
  • Have you had a reaction to other drugs in the past?

What tests will be done to diagnose a drug allergy?

Depending on the drug your allergist suspects of causing the reaction, they may suggest a skin test. Skin testing is very helpful in diagnosing an allergy to penicillin-type medicines.

During a skin prick (scratch) test, your allergist will expose your body to small amounts of specific drug allergens. They’ll clean a small area of your skin on your forearm or upper back and use a thin needle (lancet) to prick your skin with possible drug allergens.

A positive reaction usually occurs within 15 minutes of exposure to the possible allergen. Your skin may change colors (red, gray or white) and develop a raised, round spot (wheal) that looks like a mosquito bite. If you have a positive reaction, it means you may have a drug allergy.

In limited instances, providers may order a blood test. A blood test may be helpful in diagnosing a severe delayed reaction, particularly if your physician is concerned that multiple organ systems may be involved.

Management and Treatment

How is a drug allergy treated?

Your healthcare provider may recommend the following drug allergy treatments:

  • Stop taking the drug. Once the drug leaves your body, your symptoms should start to fade. If you’re taking the medication to treat a condition, your provider will prescribe another medicine that doesn’t cause an allergic reaction.
  • Antihistamines. Antihistamines block the effects of histamine in your body.
  • Corticosteroids. Corticosteroids are anti-inflammatory drugs that help reduce the production of chemicals that cause inflammation.
  • Bronchodilators. Bronchodilators are a type of medication that help relieve symptoms that affect your lungs.
  • Epinephrine auto-injector (EpiPen®). If you have severe medication allergies, your provider may prescribe this device. Epinephrine quickly reverses the symptoms of anaphylaxis. Your provider will explain when and how to use it.
  • Allergy desensitization. During allergy desensitization, a provider gives you small amounts of medications that contain the allergen. Your immune system will eventually develop a tolerance to the allergen. Your provider will gradually increase the dosage over hours or even days until your body can accept the drug without reacting.

How long does a drug allergy last?

Most people recover hours after they stop taking the medication. But in rare cases, symptoms may take weeks or even a few months to completely fade.



Can drug allergies be prevented?

If you have a drug allergy, the only way to prevent a reaction is to avoid any medications that contain the allergen. You may have to avoid similar medications, too.

Make sure healthcare providers update your medical records to indicate your drug allergy and help lower your risk of a reaction.

Outlook / Prognosis

What can I expect if I have a drug allergy?

You can’t cure a drug allergy, but you can manage your symptoms by avoiding medications you know cause a reaction and taking medicines that help reduce your immune system’s response.

Living With

How do I take care of myself?

If you have a drug allergy, always inform a healthcare provider before undergoing any type of treatment, including dental care or surgical procedures.

It’s also a good idea to carry information on you that identifies your drug allergy if you can’t or are unable to convey your allergy. Options include:

  • A health app on your smartphone. Make sure that this information is accessible from the lock screen in case of emergency.
  • Jewelry, such as a medical alert bracelet or necklace.
  • A laminated card that you keep in your wallet or purse.
  • A tattoo.

When should I see a healthcare provider?

Talk to a healthcare provider if you have allergy symptoms after taking medications.

When should I go to the ER?

Go to the emergency room or call 911 or your local emergency number immediately if you have any symptoms of a severe allergic reaction. These symptoms include:

  • Trouble breathing.
  • Difficulty swallowing.
  • Sudden weakness or lightheadedness.
  • An accelerated heartbeat.
  • Unconsciousness.

Without treatment, your symptoms may be fatal.

What questions should I ask a healthcare provider?

  • How can you tell that I have drug allergies?
  • What specific medications are causing my allergy symptoms?
  • I take this medication to treat another condition. What other medication can I safely take in its place?
  • Do I need treatment?
  • What treatment do you recommend?

A note from Cleveland Clinic

For many people, medicines help control symptoms or treat conditions. But some people can’t take certain medications because their immune system mistakes the drugs for harmful substances. Your response to a drug allergy may be different from someone else’s. A mild reaction may only cause annoying symptoms, like a drug allergy rash, congestion or watery eyes. Serious symptoms may affect your breathing or heart rate, and can even be fatal.

Contact a healthcare provider if you notice allergy symptoms after taking a medication. They can conduct tests to determine if you have drug allergies. They can also recommend treatment and alternative medications that don’t cause symptoms.

Medically Reviewed

Last reviewed on 06/06/2023.

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