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Can a familiar antibiotic help prevent STIs?

DISCO clinical trial will compare the effectiveness of taking doxycycline daily versus taking it after a potential exposure to a sexually transmitted infection.
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​A new clinical trial is set to test whether taking the antibiotic doxycycline daily or after a sexual encounter can help prevent the bacterial sexually transmitted infections (STIs) syphilis, gonorrhea and chlamydia. 

The study is co-led by Dr. Troy Grennan, medical lead for STI/HIV Services with the B.C. Centre for Disease Control (BCCDC), and involves multiple sites across the country. Recruitment has started in Vancouver, with other Canadian sites to follow, and the goal is to recruit 560 people who are likely to be exposed to STIs. 

Study participants will be randomly assigned to one of two groups, to assess whether taking the antibiotic doxycycline reduces the chances of getting an STI after having sex with someone with an infection. The first group will take doxycycline daily as a pre-exposure prophylaxis or PrEP. The second will take doxycycline only after a sexual encounter as a post-exposure prophylaxis or PEP. This is the first study globally to compare these two STI prevention approaches.

The new clinical trial comes on the heels of a research study recently published in the United States, which showed that doxycycline PEP reduced STIs by about 65 per cent, compared to a control group that did not take the medication after a sexual encounter.

Dr. Troy Grennan.jpeg
Dr. Troy Grennan is recruiting participants for the DISCO trial, to study the effectiveness of the antibiotic doxycycline in preventing STIs.

“Since we now have good data on the doxycycline PEP approach, we’ll be comparing how it measures up against doxycycline PrEP for the prevention of STIs,” said Dr. Grennan. “There have been some small studies, but this is the first large-scale trial to do a head-to-head evaluation of these different approaches to STI prevention using doxycycline.”
The randomized controlled trial is known as DISCO: Doxycycline as an Intervention for STI Chemoprophylaxis. Dr. Grennan and his colleagues are recruiting cisgender and transgender gay, bisexual, and other men who have sex with men (gbMSM), and trans women who have been treated for an STI in the past year.

People often do not experience symptoms of an STI for some time but can still pass infections on to others. For people who are more likely to be exposed to STIs, routine screening helps to detect them early so they can be treated, and most bacterial STIs are curable with antibiotics. While routine screening is an important intervention and part of sexual health care, it does not prevent the infections from occurring in the first place.  

“Rates of bacterial STIs have been increasing for well over a decade. We need new tools that are effective, safe, and acceptable to the people who use them, to help reduce the risk of infection and improve sexual health,” said Dr. Ann Burchell, a co-principal investigator based at St. Michael’s Hospital, a Unity Health site in Toronto. 

“The DISCO trial will provide important new evidence on whether the PrEP and PEP approach, so effective for HIV, can also work for other STIs.”

Dr. Grennan says along with the treatment’s effectiveness, he and his team will also be carefully monitoring antibiotic resistance. Antibiotic resistant infections are a significant threat to global health and in recent years, there have been reports of antibiotic resistant STIs. 

“Doxycycline has been around since the 1960s and we have a lot of experience using it as a longer-term treatment for acne, and prevention for malaria,” he said. “We have not seen a lot of resistance to doxycycline developing, though there isn’t a lot of data out there.”

The trial is funded by the Canadian Institutes for Health Research (CIHR) and the Canadian HIV Trials Network (CTN). Dr. Grennan’s work on this study is also supported by a Health-Professional Investigator Award from Michael Smith Health Research BC.

The trial is taking place in Toronto, Hamilton, Ottawa, Montreal, Calgary and Vancouver. If you are interested in participating, visit:



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