Drug Allergy

Allergic reactions to medicines are common. Some allergic reactions are mild. A delayed type of drug allergy that occurs 1 week or more after exposure to a medicine or vaccine is called serum sickness. A life-threatening, sudden (acute) allergic reaction that involves the whole body is called anaphylaxis.


"True" drug allergies occur when there is an allergic reaction to a medicine. This is caused by overactivity of the immune system. First, the body becomes sensitized. The immune system is triggered by your first exposure to the medicine. Following this first exposure, future exposure to the same medicine may be life-threatening.

Almost any medicine can cause an allergic reaction. Common ones are:

  • Penicillin.

  • Sulfonamides (sulfa drugs).

  • Local anesthetics.

  • X-ray dyes that contain iodine.


Common symptoms of a minor allergic reaction are:

  • Swelling around the mouth.

  • An itchy red rash or hives.

  • Vomiting or diarrhea.

Anaphylaxis can cause swelling of the mouth and throat. This makes it difficult to breathe and swallow. Severe reactions can be fatal within seconds, even after exposure to only a trace amount of the drug that causes the reaction.


  • If you are unsure of what caused your reaction, keep a diary of foods and medicines used. Include the symptoms that followed. Avoid anything that causes reactions.

  • You may want to follow up with an allergy specialist after the reaction has cleared in order to be tested to confirm the allergy. It is important to confirm that your reaction is an allergy, not just a side effect to the medicine. If you have a true allergy to a medicine, this may prevent that medicine and related medicines from being given to you when you are very ill.

  • If you have hives or a rash:

  • Take medicines as directed by your caregiver.

  • You may use an over-the-counter antihistamine (diphenhydramine) as needed.

  • Apply cold compresses to the skin or take baths in cool water. Avoid hot baths or showers.

  • If you are severely allergic:

  • Continuous observation after a severe reaction may be needed. Hospitalization is often required.

  • Wear a medical alert bracelet or necklace stating your allergy.

  • You and your family must learn how to use an anaphylaxis kit or give an epinephrine injection to temporarily treat an emergency allergic reaction. If you have had a severe reaction, always carry your epinephrine injection or anaphylaxis kit with you. This can be lifesaving if you have a severe reaction.

  • Do not drive or perform tasks after treatment until the medicines used to treat your reaction have worn off, or until your caregiver says it is okay.


  • You think you had an allergic reaction. Symptoms usually start within 30 minutes after exposure.

  • Symptoms are getting worse rather than better.

  • You develop new symptoms.

  • The symptoms that brought you to your caregiver return.


  • You have swelling of the mouth, difficulty breathing, or wheezing.

  • You have a tight feeling in your chest or throat.

  • You develop hives, swelling, or itching all over your body.

  • You develop severe vomiting or diarrhea.

  • You feel faint or pass out.

This is an emergency. Use your epinephrine injection or anaphylaxis kit as you have been instructed. Call for emergency medical help. Even if you improve after the injection, you need to be examined at a hospital emergency department.


  • Understand these instructions.

  • Will watch your condition.

  • Will get help right away if you are not doing well or get worse.