Can I prescribe azithromycin (Zithromax) to a 100-pound child?

Can You Prescribe Azithromycin to a 100-Pound Child?

Yes, you can prescribe azithromycin to a 100-pound (45 kg) child, and the dosing is straightforward: 10 mg/kg (maximum 500 mg) on day 1, followed by 5 mg/kg (maximum 250 mg) once daily for days 2-5 for most indications. 1, 2

Weight-Based Dosing for a 100-Pound Child

A 100-pound child weighs approximately 45 kg, placing them at the threshold where maximum adult dosing applies:

• For children weighing 46 kg and above, the recommended dose is 500 mg once daily 1
• For children weighing 36-45 kg, the recommended dose is 400 mg once daily 1
• Given this child is at 45 kg exactly, use the standard pediatric regimen: 450 mg (10 mg/kg) on day 1, then 225 mg (5 mg/kg) daily for days 2-5, or round to practical dosing of 500 mg day 1, then 250 mg days 2-5 1, 2

Indication-Specific Considerations

Community-Acquired Pneumonia (Atypical Pathogens)

• Azithromycin is first-line for atypical pneumonia caused by Mycoplasma pneumoniae, Chlamydia pneumoniae, or Chlamydia trachomatis 3, 1
• Use the standard 5-day regimen: 10 mg/kg (max 500 mg) day 1, then 5 mg/kg (max 250 mg) days 2-5 3, 2
• Critical pitfall: Azithromycin is NOT first-line for typical bacterial pneumonia caused by S. pneumoniae or H. influenzae—amoxicillin remains the preferred agent for these pathogens 3, 2

Streptococcal Pharyngitis

• Azithromycin is second-line only for strep throat, with penicillin or amoxicillin remaining first-line 1, 2
• Higher dosing required: 12 mg/kg once daily for 5 days (maximum 500 mg/day) due to high recurrence rates with standard 10 mg/kg regimen 1
• For a 45 kg child: 540 mg daily for 5 days (round to 500 mg for practical dosing) 1

Acute Otitis Media

• Standard 5-day regimen: 10 mg/kg day 1, then 5 mg/kg days 2-5 2
• FDA-approved indication with high-quality evidence 2

Chlamydial Infections

• For children >45 kg but <8 years: azithromycin 1 g orally as a single dose 3
• For children ≥8 years: azithromycin 1 g orally as a single dose OR doxycycline 100 mg twice daily for 7 days 3

Critical Safety Considerations

Absolute Contraindications

• Avoid azithromycin in patients with significant risk of bacteremia 4
• Do not use for suspected pneumococcal bacteremia or severe bacterial infections requiring hospitalization 3, 4

Drug Interactions

• Do not administer simultaneously with aluminum- or magnesium-containing antacids—they reduce absorption 1, 5, 2
• Monitor for QT prolongation when used with other QT-prolonging medications 1, 2
• Monitor when used with drugs metabolized by cytochrome P450 enzyme system 1, 5

Common Adverse Effects

• Gastrointestinal effects are most common but generally mild to moderate: abdominal discomfort, diarrhea, nausea, vomiting 1, 6
• Azithromycin has fewer GI side effects than erythromycin 1, 2, 6
• Treatment discontinuation due to adverse events occurs in only 1.3% of pediatric patients 7

Practical Dosing Algorithm for This 45 kg Child

Step 1: Determine the indication

• Atypical pneumonia → Standard 5-day regimen
• Strep throat → Higher dose (12 mg/kg × 5 days) and only if penicillin contraindicated
• Otitis media → Standard 5-day regimen
• Chlamydial infection → Single 1 g dose

Step 2: Calculate dose

• Standard regimen: 500 mg day 1, then 250 mg days 2-5
• Strep throat: 500 mg daily for 5 days

Step 3: Verify no contraindications

• No risk of bacteremia or severe systemic infection
• No concurrent QT-prolonging medications
• No aluminum/magnesium antacids within 2 hours

FDA-Approved Pediatric Indications

The FDA label confirms azithromycin is approved for pediatric patients for 8:

• Community-acquired pneumonia due to Chlamydophila pneumoniae, H. influenzae, Mycoplasma pneumoniae, or S. pneumoniae in patients appropriate for oral therapy
• Acute otitis media caused by H. influenzae, M. catarrhalis, or S. pneumoniae
• Pharyngitis/tonsillitis caused by S. pyogenes as an alternative to first-line therapy

Important FDA caveat: Azithromycin should not be used in pediatric patients with pneumonia who are judged inappropriate for oral therapy due to moderate to severe illness, cystic fibrosis, nosocomial infections, known/suspected bacteremia, hospitalization requirement, or significant underlying health problems 8

Resistance Concerns

• Increasing macrolide resistance is being reported with S. pneumoniae, group A streptococci, and H. influenzae 9
• Approximately 1% of azithromycin-susceptible S. pyogenes isolates become resistant following therapy 8
• This resistance pattern reinforces why azithromycin should not be first-line for typical bacterial pathogens 4, 9