FRIDAY, July 19, 2024 (HealthDay News) -- It's long been known that popping the antibiotic doxycycline within 72 hours of a risky sexual encounter can greatly reduce a person's risk for a sexually transmitted infection (STI).

In fact, the U.S. Centers for Disease Control and Prevention formally recommended this type of "morning after" strategy last month.

But what if folks at especially high risk for STIs simply took "doxy" daily -- similar to how some people now take HIV meds as pre-exposure prophylaxis (PrEP) to prevent that infection?

Two small new studies suggest that this so-called "DoxyPrEP" strategy may indeed keep STIs at bay.

Both reports are scheduled to be presented next week at the 25th International AIDS Conference in Munich.

One study involved 52 gay or bisexual Canadian men living in Toronto and Vancouver. All of the men were living with HIV, and they also had a past history of contracting a common STI, syphilis.

For 48 weeks, the men were randomly divided into two groups: One group took a 100-milligram pill of doxycycline daily, while the other took a "dummy" placebo pill. Overall, 41 of the 52 men completed the trial.

In the group that got doxycycline daily, rates of new syphilis infections fell by 79%, reported study co-author Dr Troy Grennan, of the British Columbia Centre for Disease Control.

Rates of two other common STIs also declined for the men on DoxyPrEP: gonorrhea infections fell by 68% and chlamydia infections dropped by 97%, Grennan and colleagues reported.

The second study involved another group at higher risk for STIS: sex workers.

The study tracked outcomes for 40 female sex workers in Tokyo. All were provided with daily 100 mg doses of doxycycline and followed over time.

Rates of STIs declined dramatically, according to study co-author Seitaro Abe, of Japan's National Center for Global Health and Medicine. Overall, the incidence of any new STI fell from about 232 per 100 person-years (a measure of the time an individual is tracked during a study) to 79 per 100 person-years.

The biggest improvement among the women occurred for syphilis, which declined to zero for those taking DoxyPrEP. Rates of chlamydia declined slightly and there was no difference seen when it came to the incidence of gonorrhea.

There may have been mental health benefits, as well: Nearly three-quarters of the women in the study said their fear of contracting an STI fell considerably while taking DoxyPrEP.

As to side effects,  22.7% of participants in the Tokyo study did report some nausea and/or vomiting, but otherwise there were no serious side effects from daily doxycycline use.

Abe's team believes the findings support the use of DoxyPrEP in at-risk populations.

However, larger trials may be needed, and because these findings are being presented at a medical meeting, they should be considered preliminary till published in a peer-reviewed journal.

More information

Find out more about "morning after" doxycycline use at the CDC.

SOURCE: AIDS 2024, news release, July 18, 2024