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Medication allergy or side effect?

Even though medications are supposed to make us feel better, sometimes we have a reaction to them. Many reactions to medications are recorded as “allergies” when in reality it was a side effect or intolerance but not a true allergic reaction.  Having a “drug allergy” recorded on your medical record has significant implications.  If the reaction was not an allergy, and the drug was penicillin, then you will not receive one of the best classes of antibiotics available to treat bacterial infections, even though you may tolerate it in a different formulation or dose.

A true medication allergy does not typically occur on the first exposure and is an unexpected event that occurs in a small proportion of patients and is not related to the action of the drug.  An allergy develops when the body creates allergic antibodies (IgE) to metabolites (breakdown products) of the drug that combine with blood proteins such as albumin. On a subsequent course of that drug (or a very similar drug), the IgE antibodies cross-link leading to multiple inflammatory mediators (including histamine) released from Mast cells in the skin, lungs or GI tract as well as Basophils in the blood.  These reactions occur quickly (immediate reaction within minutes), accelerated (1 hour to 3 days after taking the drug) or delayed (3 days or longer after the drug is taken). Typical allergy reactions are:

  • Immediate: hives (urticaria), swelling (angioedema), and anaphylaxis (severe allergic reaction)
  • Accelerated: hives, swelling and other rashes with fever
  • Delayed/late: various skin rashes, fever, “serum sickness” and less commonly ones that affect blood, lungs, liver, kidney or skin (vasculitis=blood vessel inflammation) or conditions resembling lupus erythematosus.

Medication side effects are the most frequent type of reaction.  They are undesirable, but frequently not avoidable.  They occur without relation to prior exposure to the medication and at the usual dose. These side effects can frequently be expected with relation to the therapy given.  Side effects include:

  • Stomach pain: aspirin, NSAIDs (ibuprofen)
  • Headache: many medications
  • Diarrhea: antibiotics
  • Rapid heart rate:  asthma inhalers like albuterol
  • Mood changes and thrush (mouth yeast infection):  steroids
  • Dry mouth and insomnia:  decongestants

Distinguishing between allergy and side effects is very important for both you and your provider.  Patients labeled as having an “allergy” may not receive the best medication possible.  If a true medication allergy exists (immediate type), and there is no alternative medication available, desensitization in the hospital may be necessary.  Unfortunately, safe, reliable, simple, predictable tests to absolutely determine drug allergy are not available except for limited medications such as penicillin.

At Family Allergy & Asthma Care of Montana, we can help determine if a true medication allergy exists.  We offer allergy skin testing to penicillin and local anesthetics and if necessary perform medication challenges in a safe environment.

Co-Authored by:  Neil Garver, Pharm. D.

Reference: Rive CM, et al.  Testing for Drug Hypersensitivity Syndromes.  Clin Biochem Rev 2013;34:15-38.

This information is solely for informational purposes and not intended as a substitute for consultation with a medical professional. 



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