What is the doxycycline (antibiotic) dose for pre-exposure prophylaxis (PrEP) against sexually transmitted infections (STIs)?

Doxycycline Dose for Pre-Exposure Prophylaxis Against STIs

The evidence does not support doxycycline for pre-exposure prophylaxis (PrEP) against STIs; the established regimen is post-exposure prophylaxis (PEP) at 200 mg taken within 72 hours after sexual exposure, with a maximum of 200 mg per 24-hour period. 1

Critical Distinction: PEP vs PrEP

The terminology in your question requires clarification, as the evidence overwhelmingly supports post-exposure rather than pre-exposure prophylaxis:

• Doxycycline PEP (post-exposure) involves taking 200 mg as a single dose within 72 hours after oral, vaginal, or anal sex, which is the CDC-recommended approach 1, 2
• Doxycycline PrEP (pre-exposure) involves taking doxycycline before sexual activity, which has minimal evidence and is not recommended in current guidelines 1
• Only one trial has evaluated doxy-PrEP, and while it showed some benefit, it was insufficient to generate formal recommendations 3

Recommended Dosing Regimen for Doxy PEP

The CDC recommends 200 mg of doxycycline (any formulation) taken as soon as possible within 72 hours after sexual exposure, not exceeding 200 mg per 24 hours. 1, 2

Key Dosing Parameters:

• Dose: 200 mg single dose 1, 4
• Timing: As soon as possible after sex, but no later than 72 hours post-exposure 1, 2
• Frequency limitation: Maximum 200 mg per 24-hour period (if multiple sexual encounters occur, do not exceed this daily maximum) 1, 2
• Duration of prescription: Provide enough doses based on anticipated sexual activity until next follow-up visit 1

Target Population

The CDC guidelines are specific about who should receive doxy PEP:

• Recommended for: Gay, bisexual, and other men who have sex with men (MSM) and transgender women (TGW) with at least one bacterial STI diagnosed in the past 12 months 1, 2
• No recommendation: Insufficient evidence for cisgender women, cisgender heterosexual men, transgender men, and other queer/nonbinary persons 1
• May consider: MSM and TGW without recent STI diagnosis but engaging in high-risk sexual activities (though not directly studied in trials) 1

Administration Instructions

To maximize efficacy and minimize side effects:

• Take on a full stomach with a full glass of liquid 1, 2
• Avoid lying down for at least 1 hour after taking to prevent esophagitis 1, 2
• Separate dose by at least 2 hours from dairy products, antacids, and supplements containing calcium, iron, magnesium, or sodium bicarbonate 1, 2

Efficacy Data

The evidence supporting doxy PEP demonstrates substantial benefit:

• Reduces syphilis and chlamydia by >70% 5
• Reduces gonorrhea by approximately 50% 5
• Meta-analysis shows reduced incidence of first STI (HR 0.538,95% CI 0.337-0.859) 3
• Particularly effective against Chlamydia trachomatis (RR 0.291,95% CI 0.093-0.911) 3

Monitoring and Follow-Up

• Assess ongoing need for doxy PEP every 3-6 months 1, 2
• Perform STI screening at baseline and every 3-6 months at anatomic sites of exposure 1, 2
• Screen for HIV per CDC HIV PrEP guidelines if not already on HIV PrEP 1

Critical Drug Interaction

Avoid doxycycline in patients taking isotretinoin or other retinoids due to risk of pseudotumor cerebri (increased intracranial pressure), which can cause permanent vision loss. 5

• If STI prophylaxis is needed while on isotretinoin, consider temporarily discontinuing isotretinoin if medically appropriate, or emphasize barrier methods and more frequent screening (every 3 months) 5
• Symptoms of pseudotumor cerebri (severe headache, visual changes, nausea, pulsatile tinnitus) can develop within days and require urgent evaluation 5

Common Pitfalls to Avoid

• Do not prescribe as daily pre-exposure prophylaxis: The evidence supports event-driven post-exposure dosing, not daily prophylactic use 1
• Do not exceed 200 mg per 24 hours: Even with multiple sexual encounters in one day, the maximum remains 200 mg 1, 2
• Do not use beyond 72 hours post-exposure: Efficacy has only been demonstrated when taken within this window 1, 2
• Do not prescribe without comprehensive sexual health services: Doxy PEP should be part of a broader approach including risk reduction counseling, STI screening, and vaccination 1