What is the purpose of taking doxycycline (antibiotic) immediately after sex?

Doxycycline Post-Exposure Prophylaxis (Doxy PEP) for STI Prevention

Doxycycline taken right after sex is used as post-exposure prophylaxis (doxy PEP) to prevent bacterial sexually transmitted infections, particularly in high-risk populations such as men who have sex with men (MSM) and transgender women (TGW) who have had a bacterial STI in the past 12 months. 1

Recommended Population and Eligibility

• CDC specifically recommends doxy PEP for:

  • Gay, bisexual, and other MSM
  • Transgender women
  • Who have had at least one bacterial STI (syphilis, chlamydia, or gonorrhea) diagnosed in the past 12 months 1, 2

• Not currently recommended for:

  • Cisgender women
  • Cisgender heterosexual men
  • Transgender men
  • Other queer and nonbinary persons 1

Dosage and Administration

• 200 mg doxycycline as a single dose
• Take as soon as possible after sexual exposure
• Must be taken within 72 hours of condomless sex
• Maximum of 200 mg per 24-hour period 1, 2
• Take with food or milk and a full glass of water
• Remain upright for at least 1 hour after taking to prevent esophageal irritation 1

Efficacy

• Reduces syphilis and chlamydia infections by >70%
• Reduces gonococcal infections by approximately 50% 1, 3
• In the DoxyPEP trial, STI incidence was reduced by:
  • 66% in MSM taking HIV PrEP (relative risk 0.34)
  • 62% in MSM living with HIV (relative risk 0.38) 4

Clinical Implementation

Initial Visit

1. Screen for STIs at anatomic sites of exposure:

   • Nucleic acid amplification test for gonorrhea and chlamydia
   • Serologic testing for syphilis
   • HIV testing for those not known to be HIV-positive 1

2. Provide counseling on:

   • Potential side effects: photosensitivity, esophagitis, GI intolerance
   • Antimicrobial resistance concerns
   • Taking with food and plenty of fluid
   • Not lying down for 1 hour after taking 1

3. Review drug interactions:

   • Separate doxycycline by at least 2 hours from:
     • Dairy products
     • Antacids
     • Supplements containing calcium, iron, magnesium, or sodium bicarbonate 1

Follow-up Visits

• Screen for STIs every 3-6 months
• Assess for side effects
• Re-evaluate need for continued doxy PEP
• Provide risk reduction counseling and condoms 1, 2

Important Considerations and Caveats

• Doxy PEP is distinct from pre-exposure prophylaxis (PrEP), which involves taking medication before exposure 1
• Should be implemented as part of a comprehensive sexual health approach 1
• Not effective for cisgender women - a randomized trial showed no significant reduction in STIs (relative risk 0.88) 5
• Concerns about antimicrobial resistance development - tetracycline-resistant gonorrhea was detected in some participants in clinical trials 4
• The Australian consensus recommends considering doxy PEP primarily for syphilis prevention in high-risk MSM 6
• Adherence may affect effectiveness - in the trial involving cisgender women, doxycycline was detected in only 29% of hair samples 5

Doxy PEP represents a novel approach to STI prevention that has shown significant efficacy in specific populations, but should be used selectively rather than as a population-level intervention due to resistance concerns and varying effectiveness across different groups.