DoxyPEP is a single dose of the antibiotic doxycycline taken after sex to reduce the chance of acquiring a bacterial STI. The UK's first national clinical guideline on its use was published by the British Association for Sexual Health and HIV (BASHH) in June 2025, making the UK one of the first countries in the world to give formal clinical direction on the approach.
The Regimen
The guidance recommends a single 200mg dose — two 100mg capsules — taken within 24 hours and no later than 72 hours after sex.
Timing is the active ingredient of the strategy. Taken promptly it can prevent an infection establishing; taken late it is much less useful.
What It Prevents
The UK guideline is specifically framed around preventing syphilis. One study published in March 2025 showed around an 80% reduction in syphilis cases among those using it.
Evidence for chlamydia prevention is also reasonably strong. Protection against gonorrhoea is considerably weaker, because gonorrhoea resistance to tetracyclines is already widespread — a point that leads directly to the main objection to the whole approach.
The Resistance Question
This is the substantive debate, and it deserves stating properly rather than dismissing.
Repeated antibiotic use in a population creates selection pressure. The concerns are that doxyPEP could drive tetracycline resistance in gonorrhoea, and could affect other bacteria people carry — including ones that cause infections far more serious than an STI.
The counterargument is that syphilis is rising, that syphilis is a serious infection with severe consequences if undetected, and that preventing infections also prevents the antibiotic courses used to treat them.
UKHSA is monitoring the position, with doxyPEP provision reported by sexual health services through routine surveillance. This is a strategy being watched carefully rather than one that has been settled.
Who It Is For
DoxyPEP is aimed at people at substantially increased risk of syphilis, and is a decision made with a sexual health service that knows your history and testing pattern. It is not something to self-prescribe, and it is not a general-population intervention.
The guideline is explicit that it should form part of a comprehensive prevention approach — alongside condoms, HIV prevention, vaccination, regular testing and prompt treatment — rather than replacing any of it.
Practical Considerations
- Photosensitivity: doxycycline causes marked sun sensitivity
- Take it upright with water, and not immediately before lying down, to avoid oesophageal irritation
- Not in pregnancy, and not in those breastfeeding
- Absorption is reduced by dairy, antacids and iron or calcium supplements
Where to Get Advice
Through a sexual health service. They can assess whether it is appropriate, discuss the resistance trade-off in your specific circumstances, and combine it with the testing schedule that should accompany it.
Buying doxycycline online to use this way, without that assessment, misses the point of a guideline that exists precisely because the decision is finely balanced.