side effect

American  

Or side-effect

noun

1. any effect of a drug, chemical, or other medicine that is in addition to its intended effect, especially an effect that is harmful or unpleasant.

2. any accompanying or consequential and usually detrimental effect.

   the side effects of air pollution.

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side effect British  

noun

1. any unwanted nontherapeutic effect caused by a drug Compare aftereffect

2. any secondary effect, esp an undesirable one

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Etymology

Origin of side effect

First recorded in 1880–85

Example Sentences

The most commonly reported side effect was a decrease in neutrophils, which are a type of white blood cell.

From Science Daily • Apr. 3, 2026

The moves are a side effect of the war with Iran, which has spread across more of the Middle East, creating tighter supplies of oil and petroleum products such as jet fuel.

From The Wall Street Journal • Apr. 1, 2026

A statement describes it as a side effect of reproductive tract disease, which she had a history of before arriving at the zoo and is common in aging female elephants.

From Los Angeles Times • Mar. 31, 2026

Partaking in them is not without risk—ozone therapy, for example, has the potential side effect of something called an air embolism, which can lead to a stroke or heart attack.

From Slate • Mar. 30, 2026

Now there’s a side effect of war I bet you never considered.

From "How I Live Now" by Meg Rosoff

Related Words

• aftereffect
• reaction