What is DoxyPEP
DoxyPEP stands for doxycycline post-exposure prophylaxis: taking a dose of an everyday antibiotic after sex, rather than before it, to lower the chance that a bacterial infection takes hold. The idea borrows from a strategy that is already familiar in sexual health, HIV post-exposure prophylaxis (PEP), but applies it to bacterial sexually transmitted infections instead of a virus.
In June 2025, the British Association for Sexual Health and HIV (BASHH) published the UK's first national guideline on DoxyPEP, specifically for the prevention of syphilis. It followed several years of trial data, mostly from the United States and France, showing that a single dose of doxycycline taken soon after condomless sex could substantially cut the risk of developing syphilis and chlamydia in people who have frequent exposure to these infections.
DoxyPEP is not a vaccine and it is not a one-off treatment course. It is used repeatedly, after each episode of condomless sex, as part of an ongoing sexual health plan that a clinician helps put together with you based on your personal risk.
The idea of using an antibiotic as prevention rather than treatment can seem unfamiliar, but the principle is the same one behind taking antimalarial tablets before travel, or antibiotics before some dental or surgical procedures: giving the body a head start against bacteria it is likely to be exposed to, rather than waiting for symptoms to appear. For syphilis and chlamydia specifically, catching the exposure early with a well-timed dose appears to make a meaningful difference to whether an infection establishes itself at all.
Who DoxyPEP is for in the UK
BASHH guidance focuses DoxyPEP on the group with the clearest, best-evidenced benefit: gay and bisexual men who have sex with men, and transgender women, who have an elevated risk of acquiring syphilis. Within this group, people who have had a bacterial STI in the last 12 months, who have multiple sexual partners, or who are part of sexual networks with higher rates of STIs are particularly likely to be considered.
For other groups, including women and heterosexual men, the evidence base is much smaller. A clinician may still consider DoxyPEP case by case if your individual risk of syphilis is judged to be high, but this is an individual decision made after discussing your circumstances, not a blanket recommendation.
DoxyPEP is not meant for everyone who is sexually active, and it is not a substitute for regular STI testing or condoms. It is one additional tool for a specific, higher-risk group.
How and when to take it
The UK dosing is straightforward: a single 200mg dose of doxycycline, taken as two 100mg capsules, after condomless sex. It works best when taken as soon as possible, ideally within 24 hours, and the evidence supports taking it up to 72 hours afterwards if an earlier dose is missed.
Doxycycline capsules should be taken with a full glass of water, and it helps to stay upright, rather than lying down straight away, as this reduces the chance of the capsule irritating the lining of the oesophagus on its way down. Taking it with food can also help settle the stomach if nausea is a problem.
There is no fixed limit on how many times DoxyPEP can be used, but it is intended for use after each episode of condomless sex with a new or casual partner, not as a daily medicine. A clinician will talk through how this fits with your actual pattern of sexual activity.
Important safety information
What DoxyPEP does and doesn't protect against
It is worth being precise about what DoxyPEP can and cannot do, because overestimating its protection is one of the main risks of using it. Trial evidence supports a meaningful reduction in the risk of syphilis and chlamydia when DoxyPEP is taken consistently after sex, though like any preventive measure it is not 100% effective.
For gonorrhoea, the picture is different. BASHH guidance is explicit that DoxyPEP is unlikely to reliably prevent gonorrhoea. This is largely because many strains of the bacteria that cause gonorrhoea in the UK are already resistant to this class of antibiotic, so the drug often has little effect on them. Relying on DoxyPEP to protect against gonorrhoea specifically is not supported by current UK guidance, and using it for that purpose risks encouraging further antibiotic resistance without a clear benefit in return.
DoxyPEP also has no effect on viral infections. It does not protect against HIV, genital herpes or genital warts. Anyone with an ongoing risk of HIV exposure should discuss separate options, such as PrEP, with a clinician, and condoms remain the most reliable way to reduce the risk of viral STIs. Regular STI testing stays just as important for anyone using DoxyPEP, since it is a way of catching and treating infections it does not prevent.
Possible side effects and things to know
Doxycycline is a well-established antibiotic and side effects from a single dose are usually mild. The most commonly reported issues are nausea, an upset stomach, and increased sensitivity to sunlight, which means strong sun exposure is best avoided for a day or two after taking a dose, or sun protection used if that isn't possible.
Taken dry or without enough water, doxycycline capsules can occasionally cause irritation or discomfort in the throat or chest, so always take it with a full glass of water and remain upright for a while afterwards rather than taking it just before bed.
As with any antibiotic used repeatedly, there is ongoing discussion in the sexual health community about the risk of broader antibiotic resistance developing over time with frequent DoxyPEP use. This is one reason UK guidance keeps the recommended group for DoxyPEP relatively narrow, and why it is reviewed regularly as new evidence becomes available.
Getting DoxyPEP in the UK
Access to DoxyPEP through NHS sexual health clinics is expanding, but it is not yet available everywhere, and provision still varies considerably by region. Some NHS sexual health services already offer it to people who meet the eligibility criteria, while others are still rolling it out.
Private and online pharmacy routes, including through a short online consultation with a UK-registered clinician, are another way to access doxycycline for this purpose where it is appropriate. Whichever route is used, DoxyPEP should always follow an individual conversation about your sexual health history and risk, not be started without that assessment.
If you think DoxyPEP might be relevant to you, the best starting point is an honest conversation with a sexual health clinic or a registered clinician, who can confirm whether it is suitable, talk through dosing and side effects, and make sure it fits sensibly alongside regular STI testing.
It also helps to think of DoxyPEP as one part of a wider routine rather than a standalone fix. Clinicians will usually want to review your STI testing schedule alongside starting DoxyPEP, since testing is what catches the infections it does not prevent, such as gonorrhoea, HIV and herpes. Being upfront about how often you are likely to need it, and about any other medicines you take, also helps your clinician check for interactions and agree a sensible supply so you are not caught without a dose when you need one.