A US National Institutes of Health (NIH)-funded study has reported that the oral antibiotic doxycycline prevented sexually transmitted infections (STIs) acquisition by two-thirds.

Researchers at the University of Washington, Seattle and the University of California at San Francisco (UCSF) led the trial.

At total of 501 adults aged at least 18 years were enrolled at four clinic sites in San Francisco and Seattle.

In the trial, 174 participants were living with HIV and 327 were taking HIV PrEP medications.

Published in the New England Journal of Medicine, the findings showed that the oral antibiotic prevented STIs in men, who took the medication within 72 hours of having condomless sex, with men and transgender women.

The post-exposure approach, which is termed as doxy-PEP, specifically resulted in a two-thirds reduction in the chlamydia, syphilis, and gonorrhea in the study participants, who were reported to have STI within the previous year.

The researchers noted that a slight increase in antibacterial resistance was found in the study that needs further exploration.

UCSF Zuckerberg San Francisco General Hospital infectious diseases professor Annie Luetkemeyer said: “Given its demonstrated efficacy in several trials, doxy-PEP should be considered as part of a sexual health package for men who have sex with men and transwomen if they have an increased risk of STIs.

“It will be important to monitor the impact of doxy-PEP on antimicrobial resistance patterns over time and weigh this against the demonstrated benefit of reduced STIs and associated decreased antibiotic use for STI treatment in men at elevated risk for recurrent STIs.”

The National Institute of Allergy and Infectious Diseases (NIAID), part of the NIH, provided fundings for the study.