Studies support the use of daily antibiotics to prevent sexually transmitted diseases in at-risk groups

It has long been known that taking antibiotics doxycycline within 72 hours of a risky sexual encounter can significantly reduce the risk of a sexually transmitted disease (STD).

In fact, the U.S. Centers for Disease Control and Prevention has formally recommended this kind of ‘morning after’ strategies last month.

But what if people at particularly high risk for sexually transmitted diseases simply took ‘doxy’ daily, just as some people now take HIV drugs as pre-exposure prophylaxis (PrEP) to prevent that infection?

Two small new studies suggest that this so-called “DoxyPrEP” strategy may indeed keep sexually transmitted infections at bay.

Both reports are expected 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 also had a history of contracting a common STD, 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 group took a “dummy” placebo pill. In total, 41 of the 52 men completed the trial.

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

Rates of two other common sexually transmitted infections also fell among men taking DoxyPrEP: gonorrhea infections fell by 68% and chlamydia infections fell by 97%, Grennan and colleagues reported.

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

The study followed the outcomes of 40 female sex workers in Tokyo, all of whom were given 100 mg of doxycycline daily and were followed over time.

According to study co-author Seitaro Abe of Japan’s National Center for Global Health and Medicine, the rates of STIs dropped dramatically. Overall, the incidence of a new STI dropped from about 232 per 100 person-years (a measure of the time an individual is followed during a study) to 79 per 100 person-years.

The greatest improvement among women was for syphilis, which dropped to zero for those taking DoxyPrEP. Rates of chlamydia dropped slightly, and no difference was seen in the incidence of gonorrhea.

There may also be mental health benefits, with nearly three-quarters of women in the study saying their fear of contracting an infection decreased. STD significantly decreased while using DoxyPrEP.

In terms of side effects, 22.7% of participants in the Tokyo study reported 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 groups.

However, larger studies may be needed and because these findings are being presented at a medical conference, they should be viewed as preliminary until published in a peer-reviewed journal.

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