Syphilis, chlamydia and gonorrhoea have been rising, and there are no vaccines against them. In the US, these infections affect gay, bisexual and other men who have sex with men and transgender women far more than other groups1. One newer way to lower the risk is doxy-PEP (often written doxy PEP or doxyPEP): a dose of the antibiotic doxycycline taken soon after sex.
It is not for everyone, and it comes with a real concern about antibiotic resistance. Here is what the guidelines in the US, UK and Australia say, and how it is taken.
What doxy-PEP is
Doxy-PEP means doxycycline post-exposure prophylaxis. You take a single 200 mg dose of doxycycline (usually two 100 mg capsules or tablets) after condomless oral, anal or vaginal sex. The US guideline says to take it within 72 hours of sex, and never more than 200 mg in 24 hours1. The UK guideline adds that it is best taken within 24 hours, and no later than 72 hours2.
If you have sex more than once over a few days, the UK guideline says you can take one 200 mg dose at the end of a 72-hour period to cover all of it, instead of several doses2.
What it helps prevent, and what it does not
In large trials among gay, bisexual and other men who have sex with men and transgender women, doxy-PEP led to clearly fewer syphilis and chlamydia infections, and a smaller drop in gonorrhoea1. Doxycycline is no longer a recommended treatment for gonorrhoea, because resistance has become common1.
Doxy-PEP does not protect against:
- HIV. Doxycycline is an antibiotic, which acts on bacteria. HIV is a virus. For HIV prevention you need PrEP, or PEP after an exposure. See our guide to PrEP vs PEP.
- Other viral infections, such as herpes, hepatitis B or C, or HPV.
- Pregnancy. It is not contraception.
Who the guidelines recommend it for
- United States (CDC, June 2024): men who have sex with men and transgender women who have had syphilis, chlamydia or gonorrhoea diagnosed in the past 12 months should be told about doxy-PEP and, after a shared decision with their provider, offered a prescription1.
- United Kingdom (BASHH, June 2025): recommended for gay, bisexual and other men who have sex with men and transgender women at higher risk of syphilis. For people assigned female at birth, clinicians may consider it case by case2.
- Australia (consensus statement, April 2024): mainly for preventing syphilis in gay, bisexual and other men who have sex with men who are at risk, usually for a limited period followed by a review3.
Why cisgender women are not routinely included
The only trial in cisgender women, carried out in Kenya, did not find a significant drop in bacterial infections1. Until there is more evidence, guidelines do not recommend it routinely for this group, although UK clinicians may consider it case by case2.
How to take it
The UK guideline gives practical advice that helps with side effects2:
- Swallow it with plenty of water and some food.
- Stay upright for 30 minutes afterwards. Doxycycline can irritate the food pipe if it sits there, so do not take it just before lying down.
- Protect your skin from the sun. Doxycycline can make you burn more easily. Avoid sunbeds and use a high-SPF sunscreen.
- Keep antacids and minerals apart. Do not take it at the same time as antacids containing aluminium, calcium or magnesium, or with zinc, iron or bismuth products. Separate them by at least 2 hours. Milk does not notably affect absorption of doxycycline.
- Mention other medicines. Some epilepsy medicines (carbamazepine, phenytoin) can make doxycycline clear faster, and it should not be used with ciclosporin or isotretinoin.
Doxy-PEP is meant to sit inside regular sexual health care. Guidelines recommend STI testing when you start and every 3 to 6 months afterwards, and checking at those visits whether you still need it1. If you get symptoms of an STI, get tested rather than relying on doxy-PEP.
Side effects, and who should not take it
The side effects most linked with doxycycline are sun sensitivity and stomach problems, including irritation or ulcers in the food pipe. Most settle when the medicine is stopped. Serious side effects were rare in the trials1. The UK guideline also lists headache, nausea, vomiting, indigestion and rash2.
Pregnancy and breastfeeding. The UK guideline suggests doxy-PEP is only used up to 15 weeks of pregnancy, because data are limited. It notes that very little doxycycline passes into breast milk and short-term use is unlikely to harm a breastfed baby, even though the product information advises against it2. Discuss either situation with a sexual health clinic first.
Anyone who has had an allergic reaction to doxycycline or any other tetracycline antibiotic must not take it. Tetracycline antibiotics can also permanently stain developing teeth, which is why they are avoided in the second half of pregnancy and in children under 84.
Antibiotic resistance
This is the main reason doxy-PEP is offered carefully rather than to everyone. Using an antibiotic regularly, rather than to treat an infection, can encourage bacteria to become resistant.
- In the US DoxyPEP study, people taking doxy-PEP carried the skin bacterium Staphylococcus aureus less often overall, but those who did carry it were more likely to carry a tetracycline-resistant strain after 12 months1.
- The Australian consensus process could not fully agree: some experts felt doxy-PEP should be used only against syphilis, because the resistance risk outweighed the benefit for other infections3.
- The UK guideline asks clinics to explain the unknowns around resistance so that people can make an informed choice2.
Our article on antibiotic resistance explains why using antibiotics only when they are needed matters for everyone.
Doxy-PEP alongside PrEP and condoms
Doxy-PEP is one tool among several. The UK and US guidelines both place it inside a wider sexual health approach: condoms, regular STI and HIV testing, vaccination, and HIV PrEP where appropriate12. The main trials were carried out among people using HIV PrEP or living with HIV2, and a PrEP check-up is a natural time to discuss doxy-PEP. See our guides to daily vs on-demand PrEP and PrEP.
We supply doxycycline, including Microdox LBX capsules. Doxy-PEP should only be used as advised by a sexual health clinic, and it needs a prescription.
Frequently asked questions
Does doxy-PEP prevent HIV?
No. Doxycycline is an antibiotic and only works against bacteria. HIV is a virus, so you need PrEP or PEP to prevent it.
What is the doxy-PEP dose?
One dose of 200 mg of doxycycline, usually two 100 mg capsules or tablets, taken within 72 hours after sex and ideally within 24 hours. Do not take more than 200 mg in 24 hours.
Can I drink alcohol with doxy-PEP?
The UK guideline notes that long-term heavy drinking may make doxycycline less effective. An occasional drink is not listed as a problem, but tell your clinic how much you usually drink.
Can I take doxy-PEP with food or milk?
Yes. Taking it with food and plenty of water helps with stomach upset, and milk does not notably affect absorption. Keep antacids, iron, zinc and calcium supplements at least 2 hours apart.
Does doxy-PEP work for women?
The only trial in cisgender women did not show a significant benefit, so guidelines do not routinely recommend it for women. In the UK, clinicians may consider it case by case.
How long does doxycycline stay in your system?
Doxycycline has a half-life of about 12 hours, which means it takes about that long for the level in the blood to halve. A single dose is mostly cleared within a few days.
Sources & references
- Bachmann LH, Barbee LA, Chan P, et al. CDC Clinical Guidelines on the Use of Doxycycline Postexposure Prophylaxis for Bacterial Sexually Transmitted Infection Prevention, United States, 2024. MMWR Recommendations and Reports. 6 June 2024;73(2):1-8.
- British Association for Sexual Health and HIV (BASHH). UK national guideline for the use of doxycycline post-exposure prophylaxis (doxyPEP). 9 June 2025.
- Australian consensus statement on doxycycline post-exposure prophylaxis (doxy-PEP) for the prevention of syphilis, chlamydia and gonorrhoea among gay, bisexual and other men who have sex with men. Medical Journal of Australia. 2024;220(7).
- US National Library of Medicine, DailyMed. Doryx MPC (doxycycline hyclate) prescribing information.
Related reading
This content is for informational purposes only and is not medical advice. Doxy-PEP is a prescription antibiotic and should be used only after discussion with a sexual health professional, together with regular STI testing. Used as prescribed by a licensed medical professional.
Lipika Sachdeva is a licensed clinical pharmacist with 20 years of experience. She holds a B.Pharm and M.Pharm from Delhi University. At OnlineMeds, she reviews medicine and health content to help ensure it is accurate, evidence-based, and easy to understand.