What the research shows
Only one study on daily doxycycline has been published: a small American pilot with 30 men living with HIV who had had syphilis more than once. Those who took 100 mg every day had fewer bacterial STIs over almost a year. That is promising, but far too small to base advice on.
Larger studies on daily doxycycline are running in Australia and Canada, with results expected in the coming years.
Why doctors do not advise it yet
- Too little evidence. One small study is not enough to know how well it works, or for whom.
- Much more antibiotic. A daily dose adds up fast, which raises the risk of resistance. See doxyPEP and antibiotic resistance.
- More side effects. Daily doxycycline means more chance of stomach complaints and sun sensitivity, every day.
- A proven alternative exists. DoxyPEP after sex works well against syphilis and chlamydia, with far less antibiotic.
Do not start a daily antibiotic on your own. Taking doxycycline every day without a doctor's advice can do more harm than good. If you have sex very often, talk to a doctor about what fits you.
What to do instead
Take doxyPEP after sex without a condom: two tablets of 100 mg together, ideally within 24 hours and at the latest 72 hours after. Keep testing every three months, and use PrEP if you need protection against HIV. Read how to take doxyPEP.
