The incidence of bacterial STIs like gonorrhea, chlamydia and syphilis is sharply increasing, particularly among men who have sex with men (MSM) and transgender women. Along with condom use, the best strategy for managing STI risks is frequent testing, and treatment with antibiotics in case of a positive test. Sometimes a person will be treated even before testing, if they learn that a previous sex partner has an STI. (This is called “empirical treatment”).

Some people who are at high risk of STIs have experimented with taking antibiotics on a regular basis to reduce the risk of infections before they happen. There is some preliminary research confirming the effectiveness of this method. However, many healthcare professionals and public health experts actively discourage this strategy for reasons explained below.

Two small trials have investigated using the antibiotic doxycycline (“doxy”) as a prevention measure. Both trials found that taking doxy before or after sex significantly reduced the risk of contracting some STIs. The researchers stress that these promising results are not a green light for broad use of these strategies, but rather suggest the need for further study.

• Doxy PrEP (pre-exposure prophylaxis -- the same term used when an HIV-negative person takes Truvada to prevent a future HIV infection) involves taking antibiotics on a daily basis. In a very small pilot study, 30 MSM took 100 mg. of doxy daily. They contracted 73% fewer cases of syphilis and 58% fewer cases of gonorrhea and chlamydia.
• Doxy PEP (post-exposure prophylaxis -- the same term used when an HIV-negative person takes HIV medications after a possible exposure to HIV, to prevent an infection from taking hold) involves taking a single dose of antibiotics after sex. In a study of 232 MSM who were part of a larger PrEP trial, participants took 200 mg. of doxy within 72 hours after sex (with a limit of three such doses per week). They contracted 73% fewer cases of syphilis and 70% fewer cases of chlamydia. There was no reduction in the rate of gonorrhea (not a surprise, since the bacteria that cause gonorrhea are now broadly resistant to doxy).

Individuals did not suffer any harm from taking the doxy. There is also some experience with patients safely and effectively using doxy on a longer-term basis to prevent conditions such as acne and malaria. However, the small size and short duration of the doxy PrEP/PEP trials leave many questions about longer term use of antibiotics to prevent STIs. Antibiotics may also affect beneficial bacteria in the digestive tract (the “gut microbiome”), though science is only just beginning to understand the role of these bacteria.

In addition, there is concern that the widespread use of antibiotics in people who do not have an infection could contribute to the growth of antibiotic resistance, a worldwide problem that poses potential threats to our ability to treat a number of bacterial infections. Since doxy is active against many bacteria, not just those that cause STIs, it is possible that regular use by a large number of people could promote resistance in other “bugs.”

The researchers who conducted the doxy PrEP/PEP trials, as well as other medical experts, have clearly stated that they do not currently recommend antibiotic prophylaxis against STIs and that further research is needed. Additional studies are planned. If individuals do decide to try these strategies, they are strongly encouraged to do so in consultation with their healthcare providers, rather than self-sourcing antibiotics and using them without medical guidance.

For some suggestions on other measures you can take to reduce your risk of contracting STIs, read "What can I do to reduce my risk of contracting STIs other than HIV?".