side effect

A side effect is an unintended, negative reaction to a medicine or treatment. If the side effect of a headache medicine is that your ears fall off, then maybe don’t take it.

In medicine, a side effect can be any reaction to a drug that's not its specific intended effect — even if it's positive. There are anti-depressants that have the side effect of suppressing the urge to smoke, and seizure disorder medications that relieve some symptoms of mental illness as well. More common are unpleasant side effects like nausea or dizziness.

Definitions of side effect
  1. noun
    a secondary and usually adverse effect of a drug or therapy
    “severe headaches are one of the side effects of the drug”
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    black tongue, furry tongue, hairy tongue
    a benign side effect of some antibiotics; dark overgrowth of the papillae of the tongue
    type of:
    a symptom caused by an illness or a drug
  2. noun
    any adverse and unwanted secondary effect
    synonyms: fallout
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    type of:
    consequence, effect, event, issue, outcome, result, upshot
    a phenomenon that follows and is caused by some previous phenomenon
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