About The Study

With the success of PrEP in HIV prevention among gay, bisexual, and other men who have sex with men (gbMSM), CTN 313 is seeking to determine whether the use of doxycycline is a feasible option for syphilis PrEP in this population. Doxycycline is an antibiotic that has been commercially available since the 1960s, and is often used to prevent malaria and treat skin conditions such as acne.

One previous pilot study has shown some promise for the use of doxycycline as syphilis PrEP; however, with the rising rate of syphilis and disproportionate impact on gbMSM living with HIV, there is an urgent need to conduct further studies to examine this question. CTN 313 (The DaDHS Trial) is a pilot, randomized placebo-controlled trial to determine the feasibility and tolerability of using daily doxycycline as syphilis PrEP in gbMSM living with HIV.

About The Disease

Up to 75% of early syphilis infections occur in gbMSM and infection rates in gbMSM living with HIV are higher than those not living with HIV. Syphilis can have a significant impact on HIV management, as syphilis may increase HIV viral load levels in those who are virologically suppressed. Genital ulcers, which form in the early stages of syphilis infection, increase the risk of HIV transmission and acquisition.

Study Approach

CTN 313 is a prospective, double-blind, randomized, controlled multi-centre pilot trial of daily doxycycline versus placebo for the prevention of syphilis among gbMSM living with HIV. The participants, who will be recruited from clinical sites in Vancouver and Toronto, will receive either daily doxycycline or placebo for 48 weeks. During this time, they will attend visits every three months to complete questionnaires on items such as syphilis PrEP acceptability, drug adherence, sexual risk behaviours, and self-reported STIs. Participants will also attend a visit at week 60 to check for any infections that may have not yet been diagnosed at week 48.

Participant safety will be assessed through clinical observation and monitoring of routine hematology and biochemistry, and the study will undergo quarterly reviews by an external study monitor, as part of the safety monitoring plan.

Eligibility Requirements


  1. Males, ≥ 18 years of age at baseline
  2. Self-reported gbMSM status
  3. Self-reported condomless anal sex with a man within the last six months
  4. Laboratory documentation of HIV-1 infection
  5. Prior diagnosis of early/infectious syphilis within preceding 36 months
  6. Able to provide informed consent

Not Allowed

  1. Known allergy or intolerance to doxycycline or tetracyclines
  2. A known diagnosis of myasthenia gravis
  3. Use of medications that could lower doxycycline levels, including barbiturates, phenytoin and carbamazepine
  4. Use of isotretinoin
  5. Any individual capable of getting pregnant

Additional Information

If you would like to take part in this study or want more information, please contact:

Study Coordinator (Vancouver)
Ramin Azmin

Study Coordinator (Toronto)
Reva Persaud
416-864-6060 x77105


Principal Investigators

Here’s who is leading this study.

Can’t find what you’re looking for? Email ctninfo@hivnet.ubc.ca.

CTN Investigator Dr. Troy Grennan
CTN Investigator Dr. Troy Grennan

Dr. Troy Grennan

Co-Leader, Co-Infections and Related Conditions Core; Member, Steering Committee

BC Centre for Disease Control

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Participating Sites

Here’s where this study is being conducted.

BCCDC Provincial STI clinic (West 12th Clinic)

655 West 12th Avenue, Vancouver, BC V5Z 4R4

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Positive Care Clinic (PCC) at St. Michael’s Hospital

4th Floor, St. Michael's Hospital, 30 Bond Street, Toronto, ON M5B 1W8

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