Scientists from Vancouver say their recent small study suggests a daily dose of a common antibiotic may prevent some infections with syphilis, gonorrhoea and chlamydia, and offer a new solution to the growing global crisis of sexually transmitted infections.
Their study, presented at the International Aids Society conference in Munich last week, was small and must be confirmed by more research, however. Questions needing to be resolved include whether STIs are likely to become resistant to the antibiotic and what effect it could have on healthy gut bacteria in people taking it every day.
The approach would be recommended primarily to people at elevated risk of sexually transmitted infections during certain periods, Dr Jeffrey Klausner, an infectious diseases physician at the University of Southern California who was not involved in the study, told The New York Times.
“The number of people who are really going to be offered this and take this is still very small,” he said. “In general, the more choices we have for people, the more prevention options we have, the better.”
The United States now has the highest rate of new syphilis infections since 1950, according to the Centres for Disease Control and Prevention (CDC). In 2022, the last year for which data are available, there were 1.6m cases of chlamydia and nearly 650 000 new cases of gonorrhoea.
Previous studies have shown that the antibiotic doxycycline substantially cuts the risk of new infections if taken within 72 hours after unprotected sex. The CDC now recommends taking doxycycline after “oral, vaginal or anal sex”.
The recommendation applies only to gay and bisexual men and transgender women who have had an STI within the previous year or who may be at risk for one. There is not enough evidence that the strategy, called doxy-PEP, works for other groups, the agency concluded.
The new study took a different tack, looking instead at preventing STIs with a daily dose of the drug, which may be better for people with frequent exposure to infections.
Some providers have suggested patients might balk at taking daily antibiotics. But many patients told the researchers it would be easier for them to remember to take a daily dose along with a daily pill for HIV prevention or treatment, said Dr Troy Grennan, who heads the HIV programme at the BC Centre for Disease Control in Vancouver and led the study.
“Healthcare providers and researchers make a lot of assumptions about what the community would prefer, and they are often wrong,” he said.
A 2015 study looked at the daily use of doxycycline before sex for preventing STIs in gay and bisexual men who have HIV, but the sample was small and there was no placebo group.
Inspired by that study, Grennan and his team tested the approach in 41 gay and bisexual men in Toronto and Vancouver. The men were already taking a daily pill to treat HIV, and they added 100 milligrams of doxycycline each day for 48 weeks.
The researchers tested the men for STIs every three months and monitored microbial resistance to the antibiotic. They found that daily doxycycline slashed the rate of syphilis infections by 79%, chlamydia by 92% and gonorrhoea by 68%.
The team has since found similar results in gay and bisexual men and trans women who do not have HIV.
Grennan said he was surprised to see that the strategy worked so well against gonorrhoea, because about 60% of cases in Canada are resistant to tetracyclines, a class of antibiotics that includes doxycycline.
A similar study in France, where the rate of gonorrhoea resistance is 65%, found that doxycycline after sex did not prevent new cases. Steady exposure to the antibiotic might be more effective than a single dose that drops off quickly, Klausner said.
Consistent with prior studies, the use of doxycycline to prevent STIs did not seem to increase microbial resistance to the antibiotic. But the study was too small to be sure, some experts warned.
The approach “required participants to take considerably more antibiotics”, said Dr Jonathan Mermin, director of the STI centre at the CDC.
Grennan and his colleagues aim to recruit 560 men and trans women with and without HIV to compare whether doxycycline taken every day is as effective as the antibiotic taken only as needed after sex.
Another study being presented this week suggests that patients who take pills to prevent HIV infection were not more likely to get STIs if they were screened twice a year instead of four times a year, as is currently recommended.
The reduced frequency may be more palatable and practical for patients, and may increase adherence to HIV preventives, the researchers said.
The study, in the Netherlands, followed 448 gay and bisexual men in Amsterdam, Rotterdam, The Hague and Nijmegen who were taking HIV preventive pills over 18 months. About half were scheduled to be screened for STIs every three months and the other half every six months.
Some participants in the six-month group sought out STI testing between the planned clinic visits. Even so, their overall visits were fewer than for those in the three-month group. Yet the rate of infections in the six-month group was only slightly higher.
Frequent STI screening is “burdensome” for many patients, especially for those who may need to take time off work or travel long distances, said Marije Groot Bruinderink, a graduate student at the University of Amsterdam who led the work.
More frequent screening also may lead to over-treatment, especially of asymptomatic cases of chlamydia and gonorrhoea that may clear on their own, Klausner noted.
Publicly-funded sexual health clinics in the US are overwhelmed and don't have enough staff or time to diagnose and treat more urgent cases of STIs, including syphilis in pregnant women, let alone asymptomatic cases, he said.
While asymptomatic infections may cause lasting harm to patients, including infertility, there's no evidence that frequent screening and treatment for chlamydia and gonorrhoea decreases rates in the population at large, Klausner added.
See more from MedicalBrief archives:
Doxycycline slashes STI risk among MSM, trans women – US study
Alarm as syphilis rates rise worldwide, including in SA