side effect

Or side-ef·fect

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.

side effect

noun

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

2. any secondary effect, esp an undesirable one

Word History and Origins

Origin of side effect¹

First recorded in 1880–85

Example Sentences

Extended treatment improved lifespan, was well tolerated, and produced no significant side effects.

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"If this translates to human patients, it's a really exciting advance. It would mean more effective chemotherapy, better response rates, and fewer side effects. That's always the goal with any sort of cancer treatment."

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And just as critically, are there any risks or unwanted side effects associated with it?

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In animal testing, no negative side effects were observed even at the highest doses.

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Understanding which brainstem areas are linked to different parts of the body may open new avenues for developing non-invasive therapies that reduce pain without widespread side effects.

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Related Words

• aftereffect 
• reaction