If taken within 72 hours of condomless sex, the common antibiotic doxycycline drastically reduces the risk of gonorrhea, syphilis and chlamydia among transwomen and gay, bisexual and other men who have sex with men, a recent study shows.

Using doxycycline like this is a form of post-exposure prophylaxis (PEP), or medicine taken after you’ve been exposed to a disease to help protect you from illness. Doxy-PEP is a new approach tested in part by Annie Luetkemeyer, MD, professor of infectious diseases at Zuckerberg San Francisco General Hospital and UCSF, alongside colleagues from the San Francisco Department of Public Health and the University of Washington.

Scientists hope that offering doxy-PEP can stem rising rates of these STIs. More than 2.5 million STI cases were diagnosed in the U.S. in 2021 alone. We asked Luetkemeyer what people need to know about doxy-PEP.

Who can take Doxy-PEP?

Doxy-Pep is an option to reduce the risk of syphilis, gonorrhea and chlamydia for transwomen as well as gay, bisexual or other men who have sex with men and who have had at least one of these STIs in the past 12 months. Clinicians can also consider prescribing doxy-PEP to these folks if they haven’t had a recent STI but if they feel they are at risk of one because they have multiple sexual partners.

In addition, clinicians can consider doxy-PEP for straight, cisgender men who are at risk of STIs, although there are no studies yet looking at use in this population. Doxy-PEP was not effective in the only study to date conducted in cisgender women so more research is needed before doxy-PEP’s use after vaginal sex is recommended.

How do I take Doxy-PEP?

Take a single dose of doxycycline as soon as possible but no more than 72 hours after having condomless sex to reduce the risk of three STIs: syphilis, gonorrhea and chlamydia. Doxy-PEP is usually given as two,100-milligram pills or one 200-milligrams tablet.

Doxycycline should be taken with a glass of water and you should not lie down for 30 minutes after taking the pills.

How well does Doxy-PEP work?

In several studies, doxy-PEP reduced the risk of gonorrhea by about 55-60% and decreased the chances of developing chlamydia and syphilis by about 80% or more.

How can I get Doxy-PEP?

You need a prescription, so speak to a health care provider. You don’t have to go to your primary care provider for a prescription, however. There are sexual health clinics that provide medicine to prevent HIV infection before or after sex, STI testing and Mpox vaccination. Many of these clinics also provide doxy-PEP.

How much does Doxy-PEP cost?

Some clinics provide doxy-PEP for free. But even if someone needs to purchase it, it’s typically pretty cheap and often covered by insurance.

Does Doxy-PEP have any side effects?

Doxy-PEP can upset your stomach and may make you more sun sensitive, so people who have fair skin or live in a very sunny climate might want to use sunscreen. Occasionally, people can get an allergic reaction or a rash to doxy-PEP.

Very rarely, people can have more serious side effects like a bad headache or vision changes. If you experience this, contact your health care provider. You should know that hearing and vision changes can be syphilis symptoms and may be a sign you should get tested.

What if I’ve had sex without a condom and think I might have been exposed to HIV?

Doxy-PEP does not protect against HIV or other viral infections, like herpes or the human papillomavirus (HPV), the virus that causes genital warts. If you think you have been exposed to HIV, you can take HIV post-exposure prophylaxis within three days to help reduce your risk of contracting HIV.

If you think you need HIV PEP, call your provider, go to an emergency room or call a local PEP hotline. In San Francisco, local HIV PEP providers include San Francisco City Clinic and the San Francisco AIDS Foundation.

It is important to know that HIV pre-exposure prophylaxis (PrEP) – HIV medication taken before sex – is a highly effective way to prevent HIV.

Can Doxy-PEP protect me from Mpox?

Doxy-PEP does not protect against Mpox. If you have unprotected sex with someone who develops Mpox symptoms or is diagnosed with Mpox, there’s a vaccine you can take up to 14 days after you may have been exposed to reduce your risk of contracting MPox.

The best prevention for Mpox is getting vaccinated before you’re exposed. The Mpox vaccine is a two-dose jab. The second dose should be given a month after the first injection. People should get both doses for the best protection.

I’ve heard it’s bad to take antibiotics when you’re not sick because it can lead to antibiotic resistance. Will taking Doxy-PEP put me at risk of developing drug-resistant STIs in the future that are harder to treat?

So far, there is no concerning data that shows the use of doxy-PEP increases drug resistance in the patients that researchers have followed for about a year. However, drug resistance is possible so scientists are continuing to monitor it.