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

One side effect for CMOs and CXOs to worry about is that customers who earn status the old-fashioned way, by actually traveling, may resent the practice of their preferred brand offering shortcuts to status. —Roger Dooley, Forbes, 23 Jan. 2025

Potential side effects include temporary darkening or lightening of the skin and persistent redness. —Laura Schober, Health, 23 Jan. 2025

For instance, if a vaccine would cause side effects in some people who get it, that could erode public trust in the vaccine. —Emily Mullin, WIRED, 22 Jan. 2025

And understanding that as a patient — yes, there are side effects around treatment, but actually there are more long-term side effects. —Simon Perry, People.com, 21 Jan. 2025

Word History

First Known Use

1814, in the meaning defined above

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