What is the recommended dosage of azithromycin (macrolide antibiotic) for a 15-year-old female weighing 57 kilograms (kg)?

Azithromycin Dosage for a 15-Year-Old Female Weighing 126 lbs (57 kg)

For a 15-year-old female weighing 57 kg, the appropriate azithromycin dosage depends on the indication, but for most respiratory tract infections, administer 500 mg orally on day 1, followed by 250 mg once daily for days 2-5. 1

Weight-Based Dosing for Adolescents

Since this patient weighs 57 kg (>46 kg), she falls into the adult dosing category:

• For respiratory tract infections (community-acquired pneumonia, atypical pneumonia): 500 mg on day 1, then 250 mg daily for days 2-5 1, 2
• Maximum dose consideration: Adolescents weighing ≥46 kg receive standard adult dosing of 500 mg maximum 1

Indication-Specific Dosing

Respiratory Tract Infections

• Community-acquired pneumonia or atypical pneumonia (Mycoplasma, Chlamydia): The standard 5-day regimen applies—500 mg on day 1, followed by 250 mg daily for days 2-5 1, 3
• This dosing achieves tissue concentrations exceeding 3 mg/kg in lung tissue, well above MIC for relevant pathogens 4

Sexually Transmitted Infections

• Uncomplicated chlamydial urethritis or cervicitis: Single 1 gram oral dose 5, 6
• This single-dose regimen is CDC-recommended and ensures compliance 5

Streptococcal Pharyngitis

• If azithromycin is used (second-line only): 12 mg/kg once daily for 5 days (maximum 500 mg/day) 1
• For this 57 kg patient: 500 mg daily for 5 days
• Important caveat: Penicillin or amoxicillin remains first-line; azithromycin has higher recurrence rates with standard dosing 1

Key Clinical Considerations

Pharmacokinetic advantages in adolescents:

• Extensive tissue distribution with concentrations 100-fold higher than serum levels 2, 4
• Terminal half-life of 68 hours allows continued efficacy for several days after completion 2, 7
• Once-daily dosing improves compliance compared to multiple-daily-dose regimens 8

Common pitfalls to avoid:

• Do not exceed adult maximum doses regardless of weight 1
• Avoid concurrent administration with aluminum- or magnesium-containing antacids, which reduce absorption 1
• Monitor for QT prolongation risk, especially if combined with other QT-prolonging agents 3
• Do not use azithromycin as first-line for typical bacterial pneumonia (S. pneumoniae, H. influenzae)—amoxicillin remains preferred 1

Expected clinical response:

• Clinical improvement should occur within 48-72 hours; if no improvement, reevaluation is necessary 3
• Gastrointestinal side effects (nausea, diarrhea, abdominal pain) are most common but generally mild to moderate 1, 8