What’s the difference between a drug allergy and a side effect?

Drug allergies and side effects present differences that are often misunderstood. They are commonly discussed  in overlapping terms that can understate  the importance in how each should be managed (Eg. “I think I’m allergic to Aspirin, it upsets my stomach”).

True medication allergies are more rare than side effects, but are potentially very serious, resulting from the activation and response of your body’s immune system to an “insult” or “foreign body”. Allergies range in severity, from mild to life-threatening, and often only occur on the second exposure to the triggering drug, food, or substance.

Mild allergic reactions might include hives, a rash, or itchy skin, while more serious ones would be throat tightness, difficulty breathing, swollen face, lightheadedness, reddened skin or blisters, or anaphylaxis (whole-body shock with life threatening low blood pressure). It’s important to seek medical attention quickly if these serious symptoms occur when taking a medicine. Before starting any new medication, be sure to let your doctor know about any drug reactions you’ve had in the past and update your list of other prescribed medications, supplements, or vitamins you may be taking.

Treatment of acute drug allergies may include antihistamines, bronchodilators, and/or corticosteroid medications to manage symptoms. Epinephrine injections may be used to treat anaphylaxis.

Medications, Supplements, and Vitamins all have an expected  range of physiologic effects when they are consumed. The desired, therapeutic effect of a drug is called its indication for use. A side effect is a known, but unwanted reaction listed in the drug labeling that may occur, even when a medication is administered correctly.1  Side effects are not uncommon and may also be mild, (eg. A runny nose) or more severe (E.g. increased risk of a heart attack). They can happen when you begin taking a new medication, supplement, or vitamin, if you suddenly stop taking one that you’ve used for some time, or if you increase or decrease the amount of the substance you take. Your age, gender, route of administration, absorption differences, kidney function, as well as co-administered medicines, foods, or supplements can all affect your likelihood of experiencing a side effect.2

Common, milder side effects of medications include headache, insomnia, gastrointestinal distress, dry mouth, blurred vision, and skin rashes. Serious side effects are more rare and might include heart rhythm disturbances, liver failure, sedation or loss of consciousness, respiratory depression, etc.

Managing side effects, in general, involves prevention and management strategies, including deciding whether the benefit of the medication outweighs the risk or annoyance of the side effect.

To summarize, the similarities between allergic reactions and side effects are that symptoms of both may range from uncomfortable to life threatening. Prevention of both include some strategies in common, such as identification and avoidance. An important difference in these is recognizing the contribution to serious allergic reactions of your immune system and tailoring treatments and prevention strategies accordingly for the best outcome.




1 https://www.fda.gov/drugs/drug-information-consumers/finding-and-learning-about-side-effects-adverse-reactions
2 https://www.merckmanuals.com/home/drugs/adverse-drug-reactions/overview-of-adverse-drug-reactions

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