'A single dose of doxycycline taken within 72 hours of unprotected sex dramatically cuts the risk of a bacterial S.T.I., studies have found. The approach seems most effective for preventing chlamydia and syphilis, and slightly less so for preventing gonorrhea.'

https://www.nytimes.com/2023/03/09/health/syphilis-chlamydia-gonorrhea-doxycycline.html?action=click&module=Well&pgtype=Homepage&section=Health