pro·​phy·​lax·​is ˌprō-fə-ˈlak-səs How to pronounce prophylaxis (audio)
 also  ˌprä-
plural prophylaxes ˌprō-fə-ˈlak-ˌsēz How to pronounce prophylaxis (audio)
 also  ˌprä-
: measures designed to preserve health (as of an individual or of society) and prevent the spread of disease

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In Greek, phylax means "guard", so prophylactic measures guard against disease by taking action ahead of time. Thus, for example, before the polio vaccine became available, prophylaxis against polio included avoiding crowds and public swimming pools. These days a well-known kind of prophylactic is used to prevent sexually transmitted diseases; but prophylactic measures only work when people use them.

Example Sentences

Recent Examples on the Web Post-exposure prophylaxis is the prevention of infection and disease in people known to be exposed to the virus. William A. Haseltine, Forbes, 5 Oct. 2022 The availability of pre-exposure prophylaxis, post-exposure prophylaxis and HIV treatment have even further decreased the fear around infection with HIV among health care providers. Lala Tanmoy Das And Eric Kutscher, CNN, 6 Aug. 2022 The primary vaccine being used in the current monkeypox outbreak for prevention and post-exposure prophylaxis is the Jynneos vaccine, administered in two doses, Dr. Passaretti said. Ana Escalante, Glamour, 5 Aug. 2022 Monkeypox vaccination can be used as pre-exposure prophylaxis (PrEP), post-exposure prophylaxis (PEP) and outbreak response PEP (PEP++), according to the CDC. Virginia Langmaid, CNN, 5 Aug. 2022 Using the vaccine in this way is known as post-exposure prophylaxis, or PEP. Benjamin Ryan, NBC News, 6 July 2022 Vaccinating someone already exposed to monkeypox to help prevent illness is called post-exposure prophylaxis or a ring vaccination strategy. Jacqueline Howard, CNN, 30 July 2022 What’s more, TPOXX could be used as post-exposure prophylaxis among people exposed to monkeypox, either among unvaccinated people or those who cannot be vaccinated, such as the immunocompromised, according to Columbia’s Zucker. Benjamin Ryan, NBC News, 22 July 2022 First approved in 2012, preexposure prophylaxis, or PrEP, given as a daily oral pill or bimonthly injection can reduce one’s chances of contracting HIV by 90 to 100 percent. David Malebranche, Scientific American, 1 June 2022 See More

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

Word History


borrowed from New Latin, from Greek prophylak-, stem of prophylássein "to keep guard before a place, be on guard, (middle voice) to be on one's guard against, take precautions against" (from pro- pro- entry 1 + phylássein "to keep watch on, guard, preserve," derivative of phylak-, phýlax "guard, guardian, protector") + -sis -sis — more at phylactery

First Known Use

1683, in the meaning defined above

Time Traveler
The first known use of prophylaxis was in 1683

Dictionary Entries Near prophylaxis

Cite this Entry

“Prophylaxis.” Dictionary, Merriam-Webster, Accessed 2 Dec. 2022.

Medical Definition



pro·​phy·​lax·​is -ˈlak-səs How to pronounce prophylaxis (audio)
plural prophylaxes -ˈlak-ˌsēz How to pronounce prophylaxis (audio)
: measures designed to preserve health and prevent the spread of disease : protective or preventive treatment
prophylaxis against viral diseases
a paste containing fluorine for dental prophylaxis

More from Merriam-Webster on prophylaxis

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