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 The new guidelines also confirm a maximum daily dose of 200 mg, and recommend reassessing a patient’s need for prophylaxis every three to six months, including regular STI testing. Annalisa Merelli, STAT, 4 June 2024 While effects against gonorrhea may be limited, and further interventions such as vaccines may be indicated, the prophylaxis has a chance of controlling the spread of chlamydia and syphilis, said Connie Celum, a professor of global health, medicine, and epidemiology at the University of Washington. Annalisa Merelli, STAT, 4 June 2024 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 See all Example Sentences for prophylaxis 

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 12 Jul. 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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