Oral Doxycycline Dosing
The standard adult dose of oral doxycycline is 200 mg on the first day (100 mg every 12 hours), followed by 100 mg daily maintenance dosing, though severe infections require 100 mg every 12 hours throughout treatment. 1
Standard Adult Dosing
- Loading dose: 200 mg on day 1 (administered as 100 mg every 12 hours) 1
- Maintenance dose: 100 mg once daily for most infections 1
- Severe infections: 100 mg every 12 hours (particularly chronic urinary tract infections) 1
Pediatric Dosing (Children ≥8 Years)
Children ≤45 kg (≤100 lbs):
- Day 1: 4 mg/kg divided into two doses (2 mg/kg every 12 hours) 1
- Subsequent days: 2 mg/kg once daily or divided into two doses 1
- Severe infections: Up to 4 mg/kg/day may be used 1
Children >45 kg (>100 lbs):
- Use standard adult dosing 1
Condition-Specific Dosing
Sexually Transmitted Infections:
- Chlamydia/NGU: 100 mg twice daily for 7 days 2, 1
- Uncomplicated gonorrhea: 100 mg twice daily for 7 days 1
- Early syphilis: 100 mg twice daily for 2 weeks 1
- Late syphilis: 100 mg twice daily for 4 weeks 1
- Doxycycline PEP for STI: Single 200 mg dose within 72 hours after sexual exposure 3
Lyme Disease:
- Single-dose prophylaxis after high-risk tick bite: 200 mg once (adults); 4 mg/kg (max 200 mg) for children ≥8 years 3
- Early Lyme disease: 100 mg twice daily for 10-21 days 2, 3
Rickettsial Diseases (Anaplasmosis, Rocky Mountain Spotted Fever):
- 100 mg twice daily for at least 3 days after fever subsides, typically minimum 5-7 days total 4
- Extend to 10-14 days if concurrent Lyme disease suspected 4
Respiratory Infections:
- Community-acquired pneumonia: 100 mg twice daily, with some experts recommending 200 mg as first dose 4
Bioterrorism/Severe Infections:
- Inhalational anthrax: 100 mg twice daily for 60 days 4, 1
- Q fever (acute): 100 mg twice daily for 14 days 3
Malaria Prophylaxis:
- Adults: 100 mg once daily 1
- Children >8 years: 2 mg/kg once daily (up to adult dose) 1
- Begin 1-2 days before travel, continue during travel and for 4 weeks after leaving malarious area 1
Critical Administration Guidelines
Timing and Food Interactions:
- Take with a full glass of water (at least 200 mL) and food to minimize GI upset 1, 5
- Remain upright for at least 1 hour after taking to prevent esophagitis 4, 5
- Separate from dairy products by at least 2 hours before AND after dosing 5
- Separate from calcium, iron, magnesium, aluminum-containing antacids/supplements by at least 2 hours 4, 5
The FDA label explicitly states that absorption is not markedly influenced by simultaneous food or milk ingestion 1, but CDC guidelines emphasize the 2-hour separation rule for dairy to prevent chelation with calcium that reduces absorption 5. This apparent contradiction is resolved by understanding that while doxycycline can be taken with food for GI tolerability, dairy specifically should be avoided in the 2-hour window.
H2-Blocker Interaction:
- Wait minimum 2 hours (preferably 3 hours) after doxycycline before administering famotidine or other H2-blockers to ensure complete absorption 5
Important Safety Considerations
- Contraindicated in children <8 years due to dental staining risk, except when benefits outweigh risks in life-threatening infections 4, 3
- Generally contraindicated in pregnancy, though may be used for life-threatening infections when benefits outweigh risks 4, 3
- Photosensitivity risk: Advise sun avoidance and UVA-absorbing sunscreen use 3, 5
- Do not exceed 200 mg per 24 hours 4
- Therapeutic serum activity persists for 24 hours after recommended dosing 1
Common Pitfalls to Avoid
- Patients often don't realize the 2-hour dairy separation applies both before AND after taking doxycycline 5
- Calcium-fortified orange juice counts as a dairy product and must be avoided 5
- Treatment delay in suspected rickettsial diseases while awaiting laboratory confirmation can lead to severe disease and death—start empirically 4
- For streptococcal infections, therapy must continue for 10 days 1