A daily dose of a widely used antibiotic can prevent infections such as syphilis, gonorrhea and chlamydia, potentially a new solution to the growing crisis of sexually transmitted infections, scientists reported Thursday.
Their study is small and needs to be confirmed by further research. Scientists still need to answer important questions, including whether STIs can become resistant to the antibiotic and what effect it might have on healthy gut bacteria in people who take it daily.
This approach would be recommended first for people at high risk of sexually transmitted infections during certain periods, said Dr. Jeffrey Klausner, an infectious diseases physician at the University of Southern California, who was not involved in the new work.
“The number of people who will actually be offered this drug and who will accept it is still very small,” he said. “In general, the more choices we give people, the more prevention options we have, the better.”
The results will be presented next week at a conference of the International AIDS Society in Munich.
The United States now has the highest rate of new syphilis infections since 1950, the Centers for Disease Control and Prevention reported in January. In 2022, the latest year for which data are available, there were 1.6 million new cases of chlamydia and nearly 650,000 new cases of gonorrhea.
Previous studies have shown that the antibiotic doxycycline significantly reduces the risk of new infections if taken within 72 hours of 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 in the past year or are at risk of developing one. The agency concluded that there is not enough evidence that the strategy, called doxy-PEP, works for other groups.
The new study took a different approach, instead looking to prevent STIs with a daily dose of the drug, which might be better for people who are frequently exposed to infections.
Some doctors have suggested that patients might be reluctant to take antibiotics every day. But many patients told researchers it would be easier for them to remember to take a daily dose at the same time as a daily pill for HIV prevention or treatment, said Dr. Troy Grennan, who directs the HIV program at the BC Centre for Disease Control in Vancouver and led the study.
“Health care 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 daily use of doxycycline before sex to prevent STIs in gay and bisexual men with HIV, but the sample size was small and there was no placebo group.
Inspired by that study, Dr. Grennan and his team tested the approach with 41 gay and bisexual men in Toronto and Vancouver. The men were already taking a daily pill to treat HIV and added 100 milligrams of doxycycline every day for 48 weeks.
The researchers tested the men every three months for STIs and monitored microbial resistance to the antibiotic. They found that taking doxycycline daily reduced the rate of syphilis infections by 79%, chlamydia by 92% and gonorrhea by 68%.
The team has since found similar results in gay and bisexual men and trans women who do not have HIV.
Dr. Grennan said he was surprised the strategy worked so well against gonorrhea because about 60 per cent of cases in Canada are resistant to tetracyclines, a class of antibiotics that includes doxycycline.
A similar study in France, where the resistance rate for gonorrhea is 65%, showed that taking doxycycline after sex did not prevent new cases. Continuous exposure to the antibiotic may be more effective than a single dose that is rapidly tapered, Dr. Klausner said.
Consistent with previous studies, using doxycycline to prevent STIs does not appear to increase microbial resistance to the antibiotic. But the study was too small to be certain, some experts cautioned.
The approach “required participants to take significantly more antibiotics,” said Dr. Jonathan Mermin, director of the CDC STI Center
Dr Grennan and colleagues want to recruit 560 trans men and 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 at the conference suggested that patients taking pills to prevent HIV infection were no more likely to get STIs if they were tested twice a year instead of four times a year, as is currently recommended.
The reduced frequency may be more acceptable and convenient for patients, and could increase adherence to HIV preventive measures, the researchers said.
The study, conducted in the Netherlands, followed 448 gay and bisexual men from Amsterdam, Rotterdam, The Hague and Nijmegen who took HIV preventative pills for 18 months. About half of them were to be tested for STIs every three months and the other half every six months.
Some participants in the six-month group did get tested for STIs between scheduled health visits. Even so, their visits were fewer than those in the three-month group. Still, the infection rate in the six-month group was only slightly higher.
Frequent STI screening is “burdensome” for many patients, especially those who have 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 can also lead to overtreatment, particularly in asymptomatic cases of chlamydia and gonorrhea that may clear up on their own, Dr. Klausner noted.
Publicly funded sexual health clinics in the United States are overwhelmed and don’t have enough staff or time to diagnose and treat the most urgent cases of STIs, including syphilis in pregnant women, let alone asymptomatic cases, he said.
Although asymptomatic infections can cause lasting harm to patients, including infertility, there is no evidence that frequent screening and treatment for chlamydia and gonorrhea lowers rates in the general population, Dr. Klausner said.