side effect

noun

: a secondary and usually adverse effect (as of a drug)

toxic side effects

called also side reaction

Examples of side effect in a Sentence

Nausea and diarrhea are the side effects of the drug.

the harmful side effects of using chemicals to kill insects

The merger created positive side effects for both companies.

Recent Examples on the Web

The site lists fatigue, mood changes, hot flashes, sleep trouble, erectile dysfunction, weight gain and bone loss as possible side effects. — Samantha Agate, Sacbee.com, 19 Oct. 2026

Bloating is often temporary and other gastric side effects may be mitigated by starting the supplement at a low dose and increasing daily consumption incrementally. — Deborah Vankin, Los Angeles Times, 19 Oct. 2026

The American Cancer Society connected her with survivors who had experienced a similar type of breast cancer and shared some of the same treatment side effects. — Roni Robbins, AJC.com, 16 Oct. 2026

That is the window in which the field’s claims about fewer side effects, meaningful efficacy at low circulating doses and consistent oral absorption either hold up or don’t. — Samantha Agate, Kansas City Star, 16 Oct. 2026

Word History

First Known Use

1814, in the meaning defined above

Rhymes for side effect

• aftereffect
• interconnect
• affect
• aspect
• bedecked
• bisect
• collect
• connect
• correct
• deflect
• detect
• direct

Kids Definition

side effect

noun

: an often harmful and unwanted effect (as of a drug) that occurs along with the basic desired effect

the environmental side effects of using chemicals to kill pests

Medical Definition

side effect

noun

: a secondary and usually adverse effect (as of a drug)

toxic side effects

a side effect of drowsiness caused by antihistamines

called also side reaction