What is Post-Exposure Prophylaxis?
Post-exposure prophylaxis (PEP) is a short course of medication you take after a possible exposure to stop an infection from taking hold. Most people know PEP from HIV care, where antiretrovirals are started within hours of a high-risk exposure and continued for several weeks. The earlier you start, the better it works.
DoxyPEP: Using an Old Antibiotic in a New Way
Doxycycline is a tetracycline antibiotic that has been around for decades, treating everything from chlamydia and syphilis to acne, pneumonia, and Lyme disease. DoxyPEP applies the same drug as a preventive dose after sex, with the goal of stopping bacterial STIs before they establish.
An early pilot study enrolled 30 MSM (men who have sex with men) living with HIV who had two or more bouts of syphilis since their HIV diagnosis. Half were randomised to daily doxycycline 100 mg for 48 weeks, and half to a financial incentive arm aimed at changing behaviour. The doxycycline group had 73% fewer bacterial STIs across any site, with no real change in their sexual behaviour. Those results opened the door to larger trials.
The strongest evidence came from the DoxyPEP randomized controlled trial (Luetkemeyer et al., NEJM 2023), which enrolled over 500 MSM and transgender women. Participants who took 200 mg of doxycycline within 72 hours of condomless sex saw a 66% reduction in chlamydia and syphilis infections compared to the control group. The drop in gonorrhea was smaller, around 55%, likely because N. gonorrhoeae already has widespread doxycycline resistance. Based on these findings, the CDC released interim clinical guidance in 2023 recommending DoxyPEP for MSM and transgender women at higher risk of bacterial STIs.
Dosage and Side Effects
The standard dose is 200 mg (two 100 mg tablets) taken once, within 72 hours of condomless sex. Earlier is better. DoxyPEP lowers the odds of infection, but doesn't drop them to zero, so routine STD testing is still part of the picture.
Doxycycline is generally well tolerated, but the common side effects are worth knowing about:
- Loss of appetite
- Nausea and vomiting
- Diarrhea
- Rash
- Sun sensitivity (sunburns happen faster)
- Hives
- Temporary tooth discolouration in younger users
Talk to a clinician before starting, and stop once you're no longer at ongoing risk of exposure.
Bottom Line
DoxyPEP is a useful tool for reducing bacterial STIs, especially chlamydia and syphilis, in people who are at higher risk. It doesn't replace condoms or testing. It adds another layer. If you're sexually active, regular testing still matters, whether or not you decide DoxyPEP is right for you.
Disclaimer: This blog post is intended for educational purposes only and should not be taken as medical advice. Always consult your healthcare provider for personal health concerns.