Medications can sometimes cause unpleasant, unwanted effects beyond their intended ones. Although people may describe themselves as being allergic to the medication they are taking, most of the problems people experience with medications are not true allergies. In fact, allergy to medication accounts for less than 13 percent of adverse reactions. The remainder of the problems fall into the category of side effects or idiosyncratic reactions (an unusual response that is peculiar to that person) to medication. For example, hives are a common side effect some people may suffer when taking narcotic pain relievers like codeine, but it is not an allergy. Certainly hives are unpleasant, perhaps unpleasant enough that you would not wish to take the medication again, but codeine does not stimulate the immune defense system to produce antibodies against codeine, so the hives and itching are not due to a true allergy. On the other hand, hives that result from taking penicillin, which may be equally unpleasant, are not just a side effect but a true allergic reaction complete with specific anti-penicillin antibodies that are detectable in the bloodstream or on a skin test. And because an allergy to penicillin is a true acute allergy, it carries the risk of developing a life-threatening reaction. Some idiosyncratic medication reactions, such as developing hives after taking codeine or other narcotics, are called pseudo-allergic reactions because they produce symptoms that mimic allergic reactions. Unlike true allergic responses, pseudoallergic responses can occur the first time you are exposed to a medication. For example, some people who take aspirin or other nonsteroidal anti-inflammatory drugs experience hives or worsening of asthma that resemble allergic reactions. It is unclear what causes these reactions, but those who experience them should avoid not only the offending medications but sometimes related medications as well. Your doctor can advise you about which medications can be effective and safe substitutes. Although it may be difficult to tell the difference between an allergic reaction, a side effect and an idiosyncratic reaction, here are some tips. - An allergic reaction to a medication never occurs on the first exposure to it, but it can occur even with small amounts of the medication at any subsequent exposure.
The reaction to a particular medication will usually be of the same type in every instance, be it a rash, itching, abdominal complaint or respiratory symptom; however, the severity of the symptoms may vary.
- A side effect is an undesirable but known action of a medication that is most often uncovered during clinical testing of the drug. Whether or not the side effect occurs often depends on the strength of the dose; the larger the dose, the more likely it will cause a side effect. That means that, unlike with a true allergy, you may not experience the unpleasant effect at a lower dose.
- An idiosyncratic reaction, unlike allergy, can occur on first exposure to the medication. And unlike a side effect, the reaction occurs only in susceptible individuals, presumably due to a genetic or metabolic abnormality.
Last Updated: 9/26/2002
The Johns Hopkins University 1996-2003. All rights reserved. This information is not intended to provide advice on personal medical matters, nor is it intended to be a substitute for consultation.
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