side effect

or side-ef·fect


noun

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.
any accompanying or consequential and usually detrimental effect: the side effects of air pollution.

Origin of side effect

First recorded in 1880–85

British Dictionary definitions for side effect

side effect

noun

any unwanted nontherapeutic effect caused by a drug Compare aftereffect (def. 2)
any secondary effect, esp an undesirable one

Medical definitions for side effect

side effect

n.

A peripheral or secondary effect, especially an undesirable secondary effect of a drug or therapeutic regimen.