Does doxycycline PEP help prevent STIs?
Medically reviewed by Andre Stone | MD, University of Pittsburgh School of Medicine on August 25th, 2026.
Yes, doxycycline as post-exposure prophylaxis (PEP) can significantly reduce certain STD infections when taken within 72 hours of unprotected sex. Clinical trials show doxycycline PEP reduces chlamydia infections by up to 88% and syphilis by 87% in high-risk populations. The CDC recommends that clinicians discuss doxycycline PEP with men who have sex with men and transgender women who were diagnosed with syphilis, chlamydia, or gonorrhea in the past 12 months. When prescribed through shared decision-making, the dose is 200 mg as soon as possible within 72 hours after sex, with no more than 200 mg in 24 hours. Doxycycline PEP is generally less effective against gonorrhea; across trials it reduced gonorrhea infections by about half, and effectiveness may vary because of differences in antibiotic resistance. Doxycycline PEP should be prescribed as part of comprehensive sexual health care, including discussion of side effects and antibiotic resistance, STI testing at exposed sites at baseline and every 3–6 months, recommended vaccinations, and other appropriate prevention measures.
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