What is the recommended dosage of doxycycline (doxy) for a 50 kg girl?

Doxycycline Dosing for a 50 kg Girl

For a 50 kg (110 lb) girl, administer doxycycline 100 mg orally every 12 hours on day 1, followed by 100 mg daily maintenance dosing for most indications. 1

Weight-Based Dosing Algorithm

Since this patient weighs over 100 pounds (45 kg), she receives adult dosing rather than pediatric weight-based calculations 2, 1:

• Day 1: 200 mg total (100 mg every 12 hours)
• Maintenance: 100 mg daily as single dose OR divided into 50 mg every 12 hours 1

For severe infections, continue 100 mg every 12 hours throughout treatment 1.

Age Considerations

The patient's age determines safety profile:

• If ≥8 years old: Doxycycline is FDA-approved with standard dosing of 100 mg twice daily for serious infections 3, 4, 1
• If <8 years old: Doxycycline can still be used when benefits outweigh risks, particularly for life-threatening infections, as short-term use (<21 days) does not cause permanent tooth staining 2, 5

Condition-Specific Adjustments

For Tickborne Rickettsial Diseases (RMSF, Ehrlichiosis)

• 100 mg every 12 hours for minimum 5-7 days total, continuing at least 3 days after fever resolves 2

For Chlamydia/Gonorrhea

• 100 mg twice daily for 7 days 3, 1

For Anthrax Post-Exposure Prophylaxis

• 100 mg twice daily for 60 days 2, 1

For Malaria Prophylaxis

• 100 mg once daily (adult dose applies for >45 kg) 1

Critical Administration Guidelines

Take with full glass of water and remain upright for 30 minutes to prevent esophageal irritation and ulceration 2. May be taken with food or milk if gastric irritation occurs, as absorption is not significantly affected 1.

Avoid within 2-3 hours of administration 5:

• Antacids containing aluminum, calcium, or magnesium
• Iron supplements
• Dairy products (if taking on empty stomach for maximum absorption)

Common Pitfalls to Avoid

• Do not underdose: At 50 kg, this patient qualifies for full adult dosing, not reduced pediatric dosing 1
• Do not confuse with other tetracyclines: Doxycycline has a superior safety profile compared to tetracycline, minocycline, or oxytetracycline, which remain contraindicated in young children 5
• Do not extend beyond necessary duration: While short courses are safe, prolonged or repeated courses should be avoided in children <8 years 5

Monitoring and Side Effects

Counsel patient on 2:

• Photosensitivity: Use sun protection and avoid direct sunlight
• GI effects: Nausea, vomiting, diarrhea (most common)
• Esophagitis risk: Take with adequate fluids while upright