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

People who are sensitive to caffeine may want to limit themselves to just a cup or two of caffeinated coffee or tea per day to avoid side effects like sleeping issues and jitteriness. —Jillian Kubala, Health, 30 Jan. 2026

As with most supplements, taking glutathione in capsule or liquid form may cause mild side effects like bloating or a stomachache. —Bestreviews, Chicago Tribune, 29 Jan. 2026

In general, pomegranate supplementation doesn’t appear to have any adverse side effects, but some individuals may have gastrointestinal sensitivities that may lead to diarrhea. —Rebeca Schiller, Verywell Health, 29 Jan. 2026

To address metabolic health conditions such as heart disease, stroke and diabetes, Hims & Hers provides micro-dose treatment plans intended to reduce side effects and promote long-term adherence. —Charles Taylor, Forbes.com, 29 Jan. 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