What is the recommended azithromycin dosage, including weight‑based dosing for children and adults, and its contraindications?

Azithromycin Dosing Recommendations

For adults, azithromycin is dosed at 500 mg on day 1, followed by 250 mg daily on days 2-5; for children ≥6 months, give 10 mg/kg (max 500 mg) on day 1, then 5 mg/kg daily (max 250 mg) on days 2-5; for infants <6 months, use 10 mg/kg daily for 5 days. 1

Weight-Based Dosing for Children

Standard 5-Day Regimen (Most Common)

For children ≥6 months with respiratory infections, community-acquired pneumonia, or pertussis 1, 2:

• Day 1: 10 mg/kg (maximum 500 mg)
• Days 2-5: 5 mg/kg daily (maximum 250 mg per day)

Practical dosing by weight using 200 mg/5 mL suspension:

• 5 kg (11 lbs): 2.5 mL day 1, then 1.25 mL days 2-5
• 10 kg (22 lbs): 5 mL day 1, then 2.5 mL days 2-5
• 20 kg (44 lbs): 5 mL day 1, then 2.5 mL days 2-5 (using 200 mg/5 mL)
• 30 kg (66 lbs): 7.5 mL day 1, then 3.75 mL days 2-5
• ≥50 kg (110 lbs): 12.5 mL day 1, then 6.25 mL days 2-5

Infants <6 Months

10 mg/kg daily for 5 consecutive days 1

This simpler regimen is critical for neonates and young infants, particularly for pertussis treatment where azithromycin is preferred over erythromycin due to lower risk of infantile hypertrophic pyloric stenosis (IHPS) 1.

Alternative Regimens for Specific Indications

Acute Otitis Media (3 options) 2:

1. Single 30 mg/kg dose (maximum 1500 mg)
2. 10 mg/kg daily for 3 days
3. Standard 5-day regimen (10 mg/kg day 1, then 5 mg/kg days 2-5)

Pharyngitis/Tonsillitis 2, 3:

• 12 mg/kg once daily for 5 days (maximum 500 mg/day)
• This higher dose (60 mg/kg total course) is superior to lower doses for Group A Streptococcal infections 3

Chlamydial Conjunctivitis 4:

• Children ≥8 years and >45 kg: 1 g single dose
• Children >45 kg but <8 years: 1 g single dose
• Neonates: 20 mg/kg/day once daily for 3 days

Adult Dosing

Standard Respiratory Infections

500 mg on day 1, followed by 250 mg daily on days 2-5 1, 5

Chlamydial Infections

1 g single oral dose 4

Community-Acquired Pneumonia (Atypical Pathogens)

• Oral: 500 mg day 1, then 250 mg days 2-5 5
• IV (if needed): 10 mg/kg on days 1-2, then transition to oral 5

Critical Contraindications and Precautions

Absolute Contraindications

• History of hypersensitivity to any macrolide antibiotic 1
• History of cholestatic jaundice or hepatic dysfunction with prior azithromycin use 1

Important Warnings

Infantile Hypertrophic Pyloric Stenosis (IHPS) 1, 4:

• Azithromycin is strongly preferred over erythromycin in infants <1 month due to lower IHPS risk
• Monitor all infants <6 weeks receiving macrolides for signs of IHPS (projectile vomiting, feeding difficulties)
• Erythromycin carries significant IHPS risk (relative risk: infinity in one cohort study) 1

Hepatic Impairment 1:

• Use with caution in patients with impaired liver function
• Monitor for signs of hepatotoxicity

Drug Interactions 1:

• Do not take simultaneously with aluminum- or magnesium-containing antacids (reduces absorption)
• Monitor patients on digoxin, triazolam, ergot alkaloids, or warfarin
• Unlike erythromycin/clarithromycin, azithromycin has fewer cytochrome P450 interactions

Cardiac Considerations 1:

• QT prolongation possible, particularly when combined with other QT-prolonging drugs
• Use caution in patients with cardiac arrhythmias

Common Pitfalls to Avoid

1. Underdosing pharyngitis: Using 30 mg/kg total course instead of 60 mg/kg (12 mg/kg × 5 days) leads to higher bacterial failure rates 3

2. Confusing suspension concentrations: The 100 mg/5 mL and 200 mg/5 mL suspensions require different volumes—always verify which concentration you're prescribing 2

3. Forgetting food interactions: Azithromycin can be taken with or without food, but antacids must be separated by at least 2 hours 1, 2

4. Using in neonates without IHPS counseling: Always warn parents of infants <6 weeks about pyloric stenosis symptoms 1

5. Inappropriate use for MRSA: Azithromycin has no activity against MRSA—vancomycin or clindamycin required 6

Pregnancy and Breastfeeding

FDA Pregnancy Category B 1

• Azithromycin is acceptable during pregnancy
• For chlamydial infections in pregnancy: 1 g single dose is preferred over doxycycline (contraindicated) 4

Adverse Effects

Most common (mild-to-moderate) 2:

• Gastrointestinal: diarrhea/loose stools (4-7%), nausea (3-5%), abdominal pain (2-5%)
• Higher rates with single 2 g dose: nausea (18%), diarrhea (14%)
• Discontinuation rate due to adverse effects: <1% 2

Rare but serious 1, 2:

• Cholestatic jaundice
• Angioedema
• Severe allergic reactions