This Affordable Antibiotic Could Prevent STIs as a Morning-After Style Pill

antibiotic for STIs
Photo Illustration by Michela Buttignol for Verywell Health; Getty Images.

Key Takeaways

  • A new study found that doxycycline, a low-cost antibiotic, resulted in a 60-70% reduction in bacterial STI transmission in men and trans women who have sex with men when taken within 72 hours of unprotected sex.
  • Doxycycline only works to prevent STIs in individuals who were born biologically male.
  • Researchers will continue to monitor the potential impact on antibiotic resistance, but so far it has proven to be a safe and effective way to reduce STI transmission.

A low-cost antibiotic that has been commonly used since the 1960s can effectively prevent bacterial sexually transmitted infections (STIs) when taken after sex, according to a new study.

The results suggest that the antibiotic doxycycline could function as a kind of morning-after pill for STIs, but it only works for individuals who were born biologically male.

The study, published in the New England Journal of Medicine, found that using doxycycline within 72 hours of unprotected sex resulted in a 60–70% reduction in syphilis, chlamydia, and gonorrhea transmission in men and trans women who have sex with men.

This isn’t the first study to demonstrate doxycycline’s effectiveness as a preventive treatment for STIs. Jeffrey Klausner, MD, MPH, a professor of clinical population and health sciences at the University of Southern California, has been researching doxycycline’s efficacy in preventing STIs since 2010. He said this new study was the final piece needed to confirm that this intervention is safe and effective.

“What we want to do in sexual health is give people choices,” Klausner told Verywell. “We need new prevention tools, and doxycycline is a very important, safe, and effective prevention tool.”

STI symptoms timeline

Illustration by Zoe Hansen for Verywell Health

The History of Using Doxycycline in Bacterial STI Prevention

In the early 2000s, Klausner was working in a clinic in San Francisco when he began to notice an increase in patients coming in with STIs, particularly among men who have sex with men. Condom use has been on the decline in this population since the introduction of HIV/AIDS treatments and preventative medications, he said.

"I started thinking about a way that we could prevent this with the use of antibiotics, similarly to the way we prevent people from getting Lyme disease and malaria with antibiotics," he said.

In 2010, in partnership with Robert Bolan, MD, at the Los Angeles Gay and Lesbian Center, Klausner launched the first randomized controlled trial to test doxycycline’s effectiveness in preventing STIs. This first pilot study resulted in a 70–75% reduction in the number of new infections of syphilis, gonorrhea, or chlamydia in those who were born biologically male and took the antibiotic daily.

But some people were concerned about the overuse of antibiotics and the potential for increased antibiotic resistance even back then, when it was a less prominent issue. So Klausner also conducted a modeling study and found that not everybody needs to take it for it to be effective at reducing transmission. Instead, only those with a very high number of sex partners, meaning 10 or more new sex partners a month, need to be on the drug for it to have an impact.

“We found that you could really just limit it to this, what we call a core population, you could have a profound impact on reducing the spread of infections in the general population,” he said.

While Klausner’s initial study looked at the impact of taking doxycycline daily, researchers in France later conducted a study on the impact of taking it within 72 hours of unprotected sex. “They also amazingly saw almost the exact same 70–75% reduction in chlamydia and syphilis,” he said.

The latest study is the third randomized controlled trial to show the same promising results.

How Doxycycline Works

Doxycycline can kill and prevent the growth of both gram-positive and -negative bacteria, according to Jennifer Bourgeois, PharmD, a clinical pharmacist and pharmacy expert at SingleCare. Its immunomodulating effects can also be used to control inflammation in certain diseases, she said.

It is often used to treat conditions including acne, skin infections, and respiratory infections, and it is also the CDC-recommended treatment for chlamydia as well as an alternative treatment for syphilis.

Some common side effects of this drug, according to Bourgeois, are mild diarrhea, photosensitivity, nausea, vomiting, skin rash/itching, headaches, and tooth discoloration, but it’s generally pretty safe.

“Doxycycline is a highly tolerated drug and has limited evidence for causing serious adverse effects,” Bourgeois said. It’s also relatively low-cost, making it accessible to a wide array of patients.

But while research shows that doxycycline is an effective method to prevent STIs in those who were born biologically male, a study done in East Africa showed that individuals who were born biologically female did not benefit in the same way.

“We don’t know if it was an issue of women not taking the medication as they were supposed to. We don’t know if there are some unique characteristics of how much doxycycline gets into the vaginal or cervical tissue, we don’t know exactly why, but it did not work in that population,” Klausner said. “So that’s why it’s really recommended for individuals who were born biologically male right now.”

The Risks of Doxycycline

In 2023, as antibiotic-resistant infections continue to rise, Klausner said this is the greatest risk associated with using doxycycline to prevent bacterial STIs.

“The benefits are clear, but we want to mitigate the risk,” Klausner said. “In this new study, there was actually no increase in antibiotic-resistant infections, but that doesn’t mean we’re going to stop monitoring and continuing to make sure that doxycycline remains safe and effective.”

Some have expressed concerns that presenting doxycycline as a kind of morning-after style pill for STIs might make people less willing to use condoms, but Klausner said condom use was already on the decline, so presenting people with further options to help limit the spread of STIs is a very positive thing.

“If people want to wear a belt and suspenders, meaning they want to use a condom and doxycycline, then that’s great,” he said. “But sometimes condoms are not available, sometimes people can lose an erection with condoms, and so on. Condoms have a lot broader protection than doxycycline, so they’re very important to promote, but I think this is another option for people who just don’t want to use or can’t use condoms.”

What This Means For You

If you were born biologically male and you often have a high number of new sexual partners, doxycycline could be an option to help reduce your chances of contracting a bacterial STI. Talk to your healthcare provider about whether you’re a good candidate for this STI “morning-after pill.”

5 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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  2. Bolan RK, Beymer MR, Weiss RE, Flynn RP, Leibowitz AA, Klausner JD. Doxycycline prophylaxis to reduce incident syphilis among HIV-infected men who have sex with men who continue to engage in high-risk sex: a randomized, controlled pilot studySex Transm Dis. 2015;42(2):98-103. doi:10.1097/OLQ.0000000000000216

  3. Wilson DP, Prestage GP, Gray RT, et al. Chemoprophylaxis is likely to be acceptable and could mitigate syphilis epidemics among populations of gay menSex Transm Dis. 2011;38(7):573-579. doi:10.1097/OLQ.0b013e31820e64fd

  4. Molina JM, Charreau I, Chidiac C, et al. Post-exposure prophylaxis with doxycycline to prevent sexually transmitted infections in men who have sex with men: an open-label randomised substudy of the ANRS IPERGAY trialLancet Infect Dis. 2018;18(3):308-317. doi:10.1016/S1473-3099(17)30725-9

  5. Stewart J, Bukusi E, Sesay FA, et al. Doxycycline post-exposure prophylaxis for prevention of sexually transmitted infections among Kenyan women using HIV pre-exposure prophylaxis: study protocol for an open-label randomized trialTrials. 2022;23(1):495. doi:10.1186/s13063-022-06458-8

By Mira Miller
Mira Miller is a freelance writer specializing in mental health, women's health, and culture.