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

And if HHS officials have concerns about mRNA vaccine side effects, that’s all the more reason to invest in their improvement.

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The researchers also found that these engineered CAR-NK cells were much less likely to induce cytokine release syndrome — a common side effect of immunotherapy treatments, which can cause life-threatening complications.

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Enhancing the drug’s skin permeability could make treatments more efficient, reduce application frequency, and possibly lower side effects related to overuse.

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It is currently used to help patients with side effects from chemotherapy.

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The drug, chemically related to vitamin B9, was typically used to ward off side effects of chemotherapy.

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Related Words

• aftereffect 
• reaction