expression

side effect

an unintended effect of a medicine

In Medicine & Health

In more detail

an unwanted effect of a medicine or treatment, especially one that is harmful or unpleasant

Examples

The medication can cause side effects such as drowsiness and nausea.
One common side effect of chemotherapy is hair loss.

How to use side effect

Side effect refers specifically to undesired results from medical treatments, drugs, or therapies. It is the standard term in medical and pharmaceutical contexts. Unlike adverse effect (more formal/technical), side effect is neutral in tone and widely used in patient information and casual discussion.

Common phrases

common side effect
serious side effect
suffer side effects
experience side effects
side effect of

Synonyms

adverse effect · adverse reaction · unintended consequence · unwanted effect · secondary effect

Word family

side-effect (noun)

A common mistake

Learners sometimes use side effect for negative consequences of non-medical things (e.g., 'a side effect of social media'); while technically understandable, the term is primarily reserved for medical contexts.

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