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What is an Adverse Effect?

Tricia Christensen
Updated May 17, 2024
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An adverse effect is an unwanted and possibly dangerous effect that occurs with medical treatment. The term is often interchanged with "side effect," but not all side effects are adverse. For example, a side effect of taking an antihistamine is drowsiness, but this isn’t necessarily unwanted, especially if that antihistamine is in a medicine used to promote sleep. The main differentiation between side effects and adverse effects is that side effects encompass all effects of a drug or treatment, dangerous or not, while adverse effects only refer to those effects that are unwanted or potentially harmful.

Any treatment can have an adverse effect. Any surgery risks things like death, infection, or other serious complications. A trip to the chiropractor could result in soreness or limited mobility. Someone seeing a therapist might become suicidal after discussing a painful past event. As with just about everything in life, any treatment action has a reaction. Some are negative and some are positive.

Medical practitioners are constantly working to minimize the adverse effects of treatment, though sometimes they must rely on treatments that have profoundly negative consequences, such as radiation therapy to attempt to cure cancer. Usually, the more treatment is needed for a condition that is life altering or life threatening, the more tolerance exists for adverse effects. Operating on the spine, which could cause paralysis and risks death, may be considered the best chance to remove a tumor or to correct a life threatening condition, for instance.

In many cases, an adverse effect isn’t something life threatening, though it may be uncomfortable. This is seen frequently in drug treatment. Drugs can have a list of uncomfortable side effects that have been reported in clinical studies. Some of these include things like stomach upset, weight changes, mood changes, headaches, pain in the rest of the body, drowsiness, dizziness, insomnia, fatigue, difficulty urinating, changes in cognition, anxiety, depression, and et cetera.

In countries where drugs are regulated, information about any adverse effect has to be made available to the public, and this becomes an interesting feature of televised advertisements for drugs, which must list many of the side effects in any advertisements. It can get discouraging to hear this list because it may sound as though every person will develop negative side effects from treatment. Actually, this is seldom true and many people have only transient experiences of side effects or never experience an adverse effect.

Many countries require that medicines be sold with a package insert that lists information about side effects. It can be helpful to know if there is any serious adverse effect to look for, but it also gives perspective to evaluate percentages of occurrence, which are often low. For other treatments, people are best off asking their medical practitioner what the types of adverse effects are and what their rate of occurrence is. For many common treatments, this kind of information is also easy to search for on the Internet.

WiseGeek is dedicated to providing accurate and trustworthy information. We carefully select reputable sources and employ a rigorous fact-checking process to maintain the highest standards. To learn more about our commitment to accuracy, read our editorial process.
Tricia Christensen
By Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a WiseGeek contributor, Tricia Christensen is based in Northern California and brings a wealth of knowledge and passion to her writing. Her wide-ranging interests include reading, writing, medicine, art, film, history, politics, ethics, and religion, all of which she incorporates into her informative articles. Tricia is currently working on her first novel.
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Tricia Christensen
Tricia Christensen
With a Literature degree from Sonoma State University and years of experience as a WiseGeek contributor, Tricia...
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