A single dose of doxycycline, a widely used antibiotic, taken after sex halved the incidence of chlamydia and early syphilis among gay and bisexual men and transgender women in San Francisco, city health officials announced on Monday. The findings offered a glimmer of hope amid a rising tide of sexually transmitted infections nationwide. In San Francisco, gay and bisexual men and transgender women who had a history of S.T.I.s or multiple sex partners were given a supply of the antibiotic and asked to take two 100-milligram pills within 72 hours of unprotected sex. New cases of chlamydia and early syphilis — but not gonorrhea — dropped over the course of about a year. “It’s not subtle, it is very fast and we’re seeing the beginning of it, not the end,” Dr. Hyman Scott, a medical director at the San Francisco Department of Public Health, said in an interview.

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Persons: —, “, Dr, Hyman Scott Organizations: Conference, San Francisco Department of Public Health Locations: San Francisco, Denver