side effect


Definition of side effect 

: 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.
See More

Recent Examples on the Web

There was some serious backlash against the Pill in 1970, based on concerns about safety and serious side effects, like embolisms and heart attacks. Carolyn Todd, Allure, "The History and Evolution of Birth Control in America," 12 July 2018 But those drugs can have dangerous and unpleasant side effects, including causing persistent C. difficile infections, which can cause chronic diarrhea and life-threatening inflammation of the colon. Karen Weintraub, STAT, "Sepsis is the third leading cause of death. Can a new blood test change that?," 3 July 2018 My mom did not suffer too many side effects, apart from fatigue. Sofia Anjuman Ali, Washington Post, "When their mom fell ill far away, these immigrant siblings faced a crisis," 30 June 2018 And, with all drugs, there are some possible side effects, like dry mouth, sore throat, dry skin, and stinging. Chloe Metzger, Marie Claire, "The FDA Approves Qbrexza Wipes to Stop Excessive Underarm Sweating​​," 29 June 2018 The trans-Atlantic trade fight has yielded an unexpected side effect: newfound appreciation in the White House for the European Union’s power on trade. Emre Peker, WSJ, "EU Sees Itself Holding the Line in Response to U.S. Metals Tariff," 28 June 2018 As any fan of American football can testify, though, video review comes with an unavoidable side effect: game delays as referees run to the sideline and take in multiple angles of plays. Tim Newcomb, Popular Mechanics, "Video Replay Is Coming to Russia’s World Cup," 8 June 2018 To have a drug available with such minor side effects, that’s a revolution in migraine. Paul Sisson,, "San Diego man says new migraine drug has ended his pain," 4 June 2018 Protectively, many of those women have received chemotherapy, which can have devastating side effects, including anemia, a weakened immune system, hair loss, diarrhea, fatigue and memory loss. Sandee Lamotte, CNN, "Many women with common breast cancer can safely skip chemo, study says," 3 June 2018

These example sentences are selected automatically from various online news sources to reflect current usage of the word 'side effect.' Views expressed in the examples do not represent the opinion of Merriam-Webster or its editors. Send us feedback.

See More

First Known Use of side effect

1814, in the meaning defined above

Keep scrolling for more

Learn More about side effect

Share side effect

Statistics for side effect

Last Updated

10 Oct 2018

Look-up Popularity

Time Traveler for side effect

The first known use of side effect was in 1814

See more words from the same year

Keep scrolling for more

More Definitions for side effect

side effect


English Language Learners Definition of side effect

: an often harmful and unwanted effect of a drug or chemical that occurs along with the desired effect

: a result of an action that is not expected or intended

side effect


Medical Definition of side effect 

: 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

Keep scrolling for more

Comments on side effect

What made you want to look up side effect? Please tell us where you read or heard it (including the quote, if possible).


to reject or criticize sharply

Get Word of the Day daily email!

Test Your Vocabulary

Words from Greek and Roman Mythology Quiz

  • the-triumph-of-venus-by-alessandro-magnasco
  • Boreal comes from the name of the ancient Greek god of which wind?
True or False

Test your knowledge - and maybe learn something along the way.

Bee Cubed

Test Your Knowledge - and learn some interesting things along the way.


Love words? Need even more definitions?

Subscribe to America's largest dictionary and get thousands more definitions and advanced search—ad free!