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

Getting too many probiotics in your diet can lead to side effects like bloating and gas. — Merve Ceylan, HEALTH, 19 Sep. 2026

The rocky planet's exceptional brightness is a side effect of its dense atmosphere, which veils the planet's surface beneath dense sulphuric acid clouds that reflect 70% of incoming sunlight back out into space, according to MIT. — Anthony Wood, Space.com, 18 Sep. 2026

In context of supination, this means minimizing the possibility of injuries; doling out a smoother, plusher ride; and kicking underpronation’s common side effects (think plantar fasciitis, ankle pain, and shin splints) to the curb. — Stacia Datskovska, Footwear News, 18 Sep. 2026

Rates of side effects — such as injection-site pain, fatigue and headaches — were more common in the mFlusiva group but still mild to moderate. — Berkeley Lovelace Jr, NBC news, 18 Sep. 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