Doxycycline Coverage and Dosing for Bacterial Infections
Antimicrobial Spectrum
Doxycycline provides broad-spectrum coverage against gram-positive and gram-negative bacteria, atypical organisms (Mycoplasma, Chlamydia, Legionella), rickettsiae, spirochetes (Borrelia, Treponema pallidum), and certain protozoa. 1, 2, 3
Key Pathogens Covered:
- Sexually Transmitted Infections: First-line for Chlamydia trachomatis, alternative for syphilis in penicillin-allergic non-pregnant patients, and effective against many Neisseria gonorrhoeae strains (though not recommended as first-line due to 77.2% tetracycline resistance in strains with elevated cefixime MICs) 1, 2, 4
- Respiratory Pathogens: Effective for community-acquired pneumonia including atypical organisms, with demonstrated efficacy comparable to fluoroquinolones 5, 3
- Intracellular Organisms: Excellent activity against Rickettsia, Ehrlichia, Mycoplasma, and Chlamydia species 3, 6, 7
- Tick-Borne Diseases: Drug of choice for Lyme disease (Borrelia) and rickettsial infections 3, 6
Standard Dosing Regimens
Adults:
- Loading dose: 200 mg on day 1 (100 mg every 12 hours) 8
- Maintenance dose: 100 mg every 12 hours for most infections 8
- Severe infections: Continue 100 mg every 12 hours rather than reducing to once daily 8
- Some experts recommend a 200 mg first dose for pneumonia to achieve adequate serum levels more rapidly 5
Pediatric Dosing (>8 years old):
- Children ≤100 lbs: 2 mg/lb divided into two doses on day 1, then 1 mg/lb daily (single or divided doses); up to 2 mg/lb may be used for severe infections 8
- Children >100 lbs: Use adult dosing 8
- Contraindicated in children <8 years (except for life-threatening infections like anthrax) due to tooth discoloration risk 1, 2
Infection-Specific Dosing
Sexually Transmitted Infections:
- Chlamydia trachomatis: 100 mg orally twice daily for 7 days 5, 1, 2, 8
- Nongonococcal urethritis (C. trachomatis or U. urealyticum): 100 mg orally twice daily for 7 days 8
- Early syphilis (penicillin allergy): 100 mg orally twice daily for 2 weeks 8
- Late syphilis (>1 year duration): 100 mg orally twice daily for 4 weeks 8
- Gonorrhea: 100 mg orally twice daily for 7 days (only as alternative second agent with ceftriaxone when azithromycin unavailable; NOT as monotherapy) 4, 8
- Acute epididymo-orchitis: 100 mg orally twice daily for at least 10 days 8
Pelvic Inflammatory Disease:
- Oral regimen: Ceftriaxone 250 mg IM once PLUS doxycycline 100 mg orally twice daily for 14 days 5
- Parenteral regimen: Ampicillin/sulbactam 3 g IV every 6 hours PLUS doxycycline 100 mg IV or orally every 12 hours; switch to oral after 24 hours of clinical improvement, continue doxycycline to complete 14 days total 5
Community-Acquired Pneumonia:
- Outpatient without comorbidities: Doxycycline 100 mg orally twice daily (consider 200 mg first dose) 5
- Duration: Continue for 10 days in streptococcal infections 8
Post-Exposure Prophylaxis (STI Prevention):
- Doxy PEP: 200 mg as single dose within 72 hours after sexual contact (maximum 200 mg per 24 hours) for MSM and transgender women with bacterial STI in past 12 months 1, 2
- Efficacy: Reduces syphilis and chlamydia by >70%, gonorrhea by ~50%; does NOT prevent Mycoplasma genitalium 1, 2
Other Infections:
- Malaria prophylaxis: 100 mg daily (adults); 2 mg/kg daily up to adult dose (children >8 years); start 1-2 days before travel, continue during and 4 weeks after 8
- Inhalational anthrax (post-exposure): 100 mg twice daily for 60 days 8
Pharmacokinetic Advantages
Doxycycline has superior pharmacokinetics compared to other tetracyclines: 75% oral bioavailability, ~12-hour half-life enabling twice-daily dosing, excellent tissue penetration due to lipophilicity, and no renal dose adjustment required (hepatic/intestinal elimination) 2, 3, 6
- Food/milk does NOT significantly impair absorption (unlike other tetracyclines), though administration with food is recommended if gastric irritation occurs 8, 3
- Administer with adequate fluids to reduce esophageal irritation and ulceration risk 8
Critical Contraindications and Precautions
Absolute Contraindications:
- Pregnancy (all trimesters) 1, 2
- Children <8 years (except life-threatening infections) due to permanent tooth discoloration 1, 2, 8
- Severe hepatic dysfunction 2, 6
Common Adverse Effects:
- Photosensitivity (advise sun protection) 1, 2
- Gastrointestinal symptoms (nausea, diarrhea) 1, 2
- Esophageal erosion/ulceration (take with fluids, avoid lying down immediately after) 1, 2, 8
Resistance Considerations:
- High tetracycline resistance in N. gonorrhoeae (77.2% in strains with elevated cefixime MICs) makes doxycycline unreliable as gonorrhea monotherapy 4
- Doxy PEP may accelerate tetracycline resistance with projected clinical lifespan of 1.6-12.1 years depending on uptake 4
- Resistance remains low in Germany and most regions for respiratory and urinary pathogens, with ~80% therapeutic success rates 6
Clinical Pearls
- Doxycycline is cost-effective: Oral therapy costs approximately $0.80/day (1995 data), making it an attractive alternative to fluoroquinolones and macrolides 6
- No accumulation in renal impairment: Safe in patients with kidney disease without dose adjustment 8, 3
- Avoid recent antibiotic class repetition: If patient recently received tetracyclines, choose different antibiotic class due to resistance risk 5
- For tubo-ovarian abscess in PID: Many providers prefer clindamycin over doxycycline for continued therapy due to superior anaerobic coverage 5