del·​e·​te·​ri·​ous ˌde-lə-ˈtir-ē-əs How to pronounce deleterious (audio)
: harmful often in a subtle or unexpected way
deleterious effects
deleterious to health
deleteriously adverb
deleteriousness noun

Did you know?

When you hold down the delete key on your keyboard or touchscreen, the effect—whoosh!—is instantaneous (unless your device is laggy, of course). Deleterious effects, however, are often not so obvious; deleterious is used to describe things that are harmful in ways that are unexpected, slow-acting, or not readily apparent. Although most often used in formal speech and writing, deleterious is far from rare. It even pops up from time to time in film and television, especially from the mouths of wonky characters, as when Seven of Nine warns the Doctor in an episode of Star Trek Voyager, “The nebula is having a deleterious effect on all the ship’s technology,” or when Higgins exclaims in the original Magnum P.I. series, “It’s shocking what a deleterious effect a regimen of nothing but mushrooms can have on a man.” We’ll take your word for it, Higgins.

Choose the Right Synonym for deleterious

pernicious, baneful, noxious, deleterious, detrimental mean exceedingly harmful.

pernicious implies irreparable harm done through evil or insidious corrupting or undermining.

the claim that pornography has a pernicious effect on society

baneful implies injury through poisoning or destroying.

the baneful notion that discipline destroys creativity

noxious applies to what is both offensive and injurious to the health of a body or mind.

noxious chemical fumes

deleterious applies to what has an often unsuspected harmful effect.

a diet found to have deleterious effects

detrimental implies obvious harmfulness to something specified.

the detrimental effects of excessive drinking

Examples of deleterious in a Sentence

In developing countries, the imposition of boundaries around national parks and protected areas has been deleterious for both people and wildlife. Kristin B. Gunther, Association of American Geographers, 2002
Most everyone now knows the action of pressing mouse keys is far more deleterious to the tender structures of the wrist and hand than typing is. Michael Finley, Albany (New York) Times-Union, 2 Sept. 1998
This skewed allocation of resources away from those most in need, in turn, insures that many young people from low-income families will continue to be shut out of the economy, with obvious deleterious family and social consequences. Iris C. Rotberg, Education Week, 9 Mar. 1994
For as James Keirans pointed out to me, ticks are the world's most notorious carriers of the agents of diseases that strike animals, wild and domestic. And they rank second only to mosquitoes in their deleterious effect on humans. Edwards Park, Smithsonian, January 1987
The chemical is deleterious to the environment. The drug has no deleterious effects on patients.
Recent Examples on the Web Read More: How COVID-19 Vaccines and Infections Are Tweaking Our Immunity An extremely similar deleterious antibody response occurs in susceptible patients after infection with adenoviruses, which often infect the airways and lead to cold-like symptoms, the study found. Jason Gale / Bloomberg, TIME, 16 May 2024 Advertisement Over the last year, California’s employment growth has been lagging behind the nation as a whole, in large part due to the deleterious effects of high interest rates on three pillars of the state’s economy — high-tech, entertainment and housing. Don Lee, Los Angeles Times, 19 Apr. 2024 The zinc concentrate is expected to be a marketable concentrate with no deleterious elements other than an iron penalty. Fort Worth Star-Telegram, 28 Feb. 2024 The deleterious effect remote learning was having not just on student performance but also on young people’s mental health was observable and, indeed, chronicled at the anecdotal and clinical level. The Editors, National Review, 20 Mar. 2024 See all Example Sentences for deleterious 

These examples are programmatically compiled from various online sources to illustrate current usage of the word 'deleterious.' Any opinions expressed in the examples do not represent those of Merriam-Webster or its editors. Send us feedback about these examples.

Word History


Greek dēlētērios, from dēleisthai to hurt

First Known Use

1587, in the meaning defined above

Time Traveler
The first known use of deleterious was in 1587


Dictionary Entries Near deleterious

Cite this Entry

“Deleterious.” Dictionary, Merriam-Webster, Accessed 13 Jun. 2024.

Kids Definition


del·​e·​te·​ri·​ous ˌdel-ə-ˈtir-ē-əs How to pronounce deleterious (audio)
deleteriously adverb
deleteriousness noun

Medical Definition


del·​e·​te·​ri·​ous ˌdel-ə-ˈtir-ē-əs How to pronounce deleterious (audio)
: harmful often in a subtle or an unexpected way
the deleterious effects of radiation and chemotherapy on the marrowChristine Gorman
deleterious genes

More from Merriam-Webster on deleterious

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