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.

Examples of prophylaxis in a Sentence

Recent Examples on the Web Some antivirals are also approved for post-exposure prophylaxis, meaning that they are given to people who have not yet developed the illness but have had exposure to the virus. Katia Hetter, CNN, 11 Apr. 2024 People exposed to measles who cannot document immunity against the virus should get post-exposure prophylaxis — a dose of the vaccine to potentially provide protection within 72 hours of initial exposure, or immunoglobulin within six days of exposure. Kristen Jordan Shamus, Detroit Free Press, 10 Apr. 2024 Developing long-acting ways of treating HIV has become a major focus in efforts to manage the virus, including to prevent infection, known as pre-exposure prophylaxis, or PrEP. Robert Hart, Forbes, 21 Feb. 2024 The patient should then be tested, post-exposure prophylaxis should be provided to close contacts and all contacts who are not up to date on their measles vaccinations should be vaccinated. Mary Kekatos, ABC News, 27 Jan. 2024 People who were possibly exposed received post-exposure prophylaxis immune globin, using plasma containing antibodies to immediately protect against measles. Eduardo Cuevas, USA TODAY, 9 Jan. 2024 InMOTION also partners with community health workers to identify people who do not have HIV and are eligible to receive pre-exposure prophylaxis (PrEP), which helps prevent the virus, said Springer. Anika Nayak, STAT, 10 Nov. 2023 This continuum of datapoints highlights the neglected and growing need for prophylaxis of diarrhea in cancer patients. Miami Herald, 25 Jan. 2024 Intravenous magnesium used as a prophylaxis turned out to be the answer. Rachel Nuwer, Scientific American, 5 Jan. 2024

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

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 20 May. 2024.

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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