Side effect can mean:
- Adverse drug reaction, an unintended consequence specifically arising from drug therapy
- Therapeutic effect, a desirable consequence of any kind of medical treatment, even though resulting as an unintended, unexpected consequence of the treatment
- Adverse effect (medicine), an unintended, and undesirable, consequence of any kind of medical treatment
- Unintended consequence, an effect that is unforeseen, regardless of type or quality