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

Like pretty much all vaccines and treatments, mRNA vaccines are not without side effects, but evidence shows that any adverse events are nearly always mild and short-lived. —Simon Williams, Time, 15 Aug. 2025

Although bananas can fit easily into a balanced diet, having too many may lead to side effects ranging from digestive discomfort to more serious health concerns.34 1. —Lindsay Curtis, Verywell Health, 13 Aug. 2025

Their reasons include: Significant side effects: Benadryl causes drowsiness, delayed reaction time, and confusion. —Emily Kay Votruba, EverydayHealth.com, 13 Aug. 2025

Although initial safety profiles are encouraging and side effects manageable, ongoing studies are exploring methods to reduce or eliminate immune suppression, such as gene editing and immune cloaking techniques. —William A. Haseltine, Forbes.com, 12 Aug. 2025

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