What organism does doxycycline (Doxycycline) treat in sexually transmitted diseases (STDs)?

Doxycycline in STDs: Target Organisms

Doxycycline is used to treat three primary bacterial sexually transmitted infections: Chlamydia trachomatis, Treponema pallidum (syphilis), and Neisseria gonorrhoeae (gonorrhea), with the highest efficacy against chlamydia and syphilis. 1

Primary Target Organisms

Chlamydia trachomatis (Most Effective)

• Doxycycline 100 mg orally twice daily for 7 days is first-line treatment for uncomplicated chlamydial infections, with 97-98% cure rates 2, 3
• Effective for uncomplicated urethral, endocervical, and rectal infections caused by C. trachomatis 4
• Also treats lymphogranuloma venereum, trachoma, inclusion conjunctivitis, and nongonococcal urethritis caused by Ureaplasma urealyticum 4
• The CDC updated recommendations in 2021 from single-dose azithromycin to 7-day doxycycline as preferred treatment 5

Treponema pallidum (Syphilis)

• Doxycycline is an alternative treatment for syphilis in nonpregnant patients with severe penicillin allergy or when penicillin is unavailable 1
• When used as postexposure prophylaxis (doxy PEP), reduces syphilis incidence by approximately 77-87% 1, 6
• Indicated when penicillin is contraindicated 4

Neisseria gonorrhoeae (Gonorrhea - Limited Efficacy)

• Doxycycline is NOT a recommended treatment for gonorrhea due to elevated antimicrobial resistance, but remains effective against many strains in the United States 1
• When used as postexposure prophylaxis, reduces gonorrhea incidence by approximately 50-57% 1, 6
• Historically used for uncomplicated gonorrhea when penicillin was contraindicated, but resistance patterns have changed 4

Novel Prophylactic Use (Doxy PEP)

CDC 2024 Recommendations for Prevention

• The CDC recommends doxycycline 200 mg taken within 72 hours after condomless sex (doxy PEP) for MSM and transgender women who had a bacterial STI diagnosed in the past 12 months 1
• Reduces combined incidence of syphilis, chlamydia, and gonorrhea by approximately two-thirds compared to standard care 6
• Maximum dose: 200 mg every 24 hours 1
• Should be offered following shared decision-making with providers 1

Efficacy by Organism in Prevention Studies

• Chlamydia: 74-88% reduction 6
• Syphilis: 77-87% reduction 6
• Gonorrhea: 50-57% reduction 6

Important Clinical Considerations

Coinfection Patterns

• When gonorrhea is confirmed, always treat for chlamydia concurrently due to 20-40% coinfection rates 3
• Patients diagnosed with any bacterial STI should be tested for gonorrhea, syphilis, and HIV at initial visit 2

Resistance Concerns

• Tetracycline-resistant gonorrhea occurred in 5 of 13 participants using doxycycline prophylaxis versus 2 of 16 in standard care groups in recent trials 6
• Despite in vitro resistance reports for C. trachomatis, clinical significance remains unknown 7
• Microbiologic failure has been documented even after prolonged doxycycline courses 7

Contraindications and Special Populations

• Absolutely contraindicated in pregnancy 2
• For pregnant women with chlamydia: use azithromycin 1 g single dose or amoxicillin 500 mg three times daily for 7 days 2, 3
• Common adverse effects include photosensitivity and gastrointestinal symptoms including esophageal erosion 1

Partner Management

• All sex partners from the previous 60 days must be evaluated, tested, and empirically treated 2, 8
• Patients must abstain from sexual intercourse for 7 days after initiating treatment and until all partners complete treatment 2, 3
• Failure to treat partners leads to reinfection in up to 20% of cases 2