A single dose of doxycycline taken within three days of condomless sex cuts the risk of gonorrhoea, chlamydia and syphilis diagnoses by two thirds among high-risk patients, a new study shows.

US researchers say their findings support the use of doxycycline post-exposure prophylaxis — or doxy-PEP — as a morning after pill for STIs in men who have sex with men (MSM) and transgender women.

“These results showed effectiveness of doxy-PEP … regardless of HIV status in a socioeconomically and racially diverse population,” they concluded in The New England Journal of Medicine.

The investigators tracked STI incidence in 501 adults (median age 38) from four clinics in San Francisco and Seattle who were randomly assigned in a 2:1 ratio to a 200mg oral dose of doxycycline within 72 hours of condomless anal or vaginal sex, or standard care without doxycycline.