What is a drug allergy?

A drug allergy is an adverse reaction to a medication. Drug allergies occur when the immune system overreacts to a medication.

Medications such as antibiotics, non-steroidal anti-inflammatory medications, anticonvulsants, chemotherapy and monoclonal antibody therapy are most likely to cause a reaction. However, any medication (OTC, prescription or herbal) can be at fault.

Side effects of the medication are sometimes mistaken for drug allergies. True drug allergies occur in less than ten percent of reactions.

Causes of a drug allergy

Drug allergies occur when your body’s immune system mistakes a medication as a foreign invader such as a virus, bacteria or other dangerous substance.

After taking a medication that you are allergic to, the body will start producing antibodies to fight the invader. This could lead to inflammation, which could cause symptoms such as fever, rash or trouble breathing.

An allergic reaction to a medication may not happen until after you have taken the medication a few times.

Risk factors of a drug allergy

Though any person could have a drug reaction to a medication, factors that increase your chance of having an allergic reaction include:

  • History of allergies
  • Family history of drug allergies
  • History of allergic reactions to medications
  • Increased exposure to medications at high doses

Symptoms of a drug allergy

Although symptoms of drug allergies appear one hour after ingesting the medication, some reactions can occur many hours, days or even weeks later.

The most common symptoms of a drug allergy are:

  • Hives
  • Rash
  • Fever
  • Runny nose
  • Fever
  • Swelling
  • Shortness of breath
  • Wheezing
  • Itchy, watery eyes

In serious cases, a drug allergy can cause anaphylaxis. Anaphylaxis is a serious, potentially life-threatening reaction to a medication that could cause disruption throughout the body.

Diagnosis of a drug allergy

It is challenging to diagnose a drug allergy. If your primary care doctor suspects a drug allergy, he or she may refer you to an allergist for further testing and treatment.

During your exam, your allergist will ask you a variety of questions including:

  • What is the suspected drug that caused the reaction?
  • When did you take it?
  • Have you stopped taking it?
  • When did the symptoms occur in relation to when you took the medication?
  • What were your symptoms and how long did they last?Did you take any other medications around the same time?

Depending on the suspected cause of the reaction, your doctor may order a skin test to confirm. Although skin tests can indicate penicillin allergies, some allergic reactions to drugs can resemble other conditions.

In some cases, your doctor may order a “drug challenge,” where you will be given different medications to determine what is triggering the reaction.

Treatment for a drug allergy

Treatment for a drug allergy will depend upon how allergic you are to the medication.

If you only have a mild reaction and the doctor cannot exchange the medication for a different one, he or she may continue to prescribe it to you and add another medication to block the immune response to the medication that causes the reaction.

Medications that can block the immune response include:
  • Antihistamines — can block the production of histamine (substance that triggers allergic reactions).
  • Corticosteroids — can reduce inflammation that could potentially restrict the airway.
  • Bronchodilators — help open the airways to make breathing easier.

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