Is it a side effect or drug allergy?

Mar 11, 2022 | Marie Suszynski, Writer | @mariemayohecs

Medications can cause negative effects even while working well. But there’s a difference between a side effect — a potentially expected, albeit unpleasant, reaction to a drug — and a drug allergy — a reaction of your immune system.

Incorrectly calling a side effect an allergy might cause a drug or related drugs to be flagged in your medical record, preventing providers from prescribing an otherwise appropriate medication.

Here are some tips to know the difference:

  • Check the documentation — While drug labels include a brief list of side effects, the printed materials that come with the drug contain a full list. Or you can view side effects at
  • Know your risk — People at a higher risk of drug allergies tend to have a history of other allergies, including a food allergy or hay fever. They also tend to have a personal or family history of drug allergy. People with drug allergies also may have had increased exposure to a drug because of high doses, repetitive use or prolonged use. And certain illnesses, such as infection with HIV or the Epstein- Barr virus, are commonly linked to allergic drug reactions. Keep in mind, however, that drug treatments may cause side effects, such as rashes, that mimic allergic reaction.
  • Document your reactions — A drug can cause side effects in one person and an allergy in another. A prime example is penicillin. Severe symptoms, such as hives or throat swelling, may signal an allergic reaction. Milder problems, such as itching or a bloated feeling, are more likely side effects. Document all reactions, including how long after taking a drug they occur.
  • Talk to your provider — Call your doctor or pharmacist. He or she is trained and experienced in differentiating between the two.


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