Azithromycin (Zithromax) Prophylactic Dosing
For postexposure prophylaxis of pertussis, azithromycin is administered as a 5-day course, NOT as a single dose: adults receive 500 mg on day 1 followed by 250 mg daily on days 2-5, while infants under 6 months receive 10 mg/kg daily for 5 days, and children over 6 months receive 10 mg/kg (max 500 mg) on day 1 followed by 5 mg/kg daily (max 250 mg) on days 2-5. 1
Age-Based Dosing Algorithm
Infants <6 months:
- 10 mg/kg per day for 5 days 1
- This is the preferred macrolide for neonates (<1 month) due to lower risk of infantile hypertrophic pyloric stenosis (IHPS) compared to erythromycin 1
- Monitor all infants <1 month receiving macrolides for IHPS and other serious adverse events 1
Children ≥6 months:
- Day 1: 10 mg/kg (maximum 500 mg)
- Days 2-5: 5 mg/kg per day (maximum 250 mg) 1
Adults:
- Day 1: 500 mg
- Days 2-5: 250 mg per day 1
Critical Context: This is NOT a Single-Dose Regimen
The question asks about "protection dose" which may imply a single dose, but azithromycin prophylaxis for pertussis requires a 5-day course. The CDC guidelines are explicit that this is administered as a single daily dose over 5 days, not as a one-time prophylactic dose 1.
Important Safety Considerations
For neonates (<1 month): Azithromycin is strongly preferred over erythromycin because erythromycin carries significant risk of IHPS (7 cases out of 157 exposed infants in one cohort study) 1. The high mortality risk of pertussis in this age group outweighs the potential risks of azithromycin 1.
Drug interactions to avoid:
- Do not administer with aluminum- or magnesium-containing antacids simultaneously (reduces absorption) 1
- Monitor patients on CYP450-metabolized drugs (digoxin, triazolam, ergot alkaloids) 1
- Use caution in patients with hepatic impairment 1
Common adverse effects: Abdominal discomfort, diarrhea, nausea, vomiting, headache, and dizziness 1
Alternative Single-Dose Context
If the question refers to chlamydial infections (not pertussis), then a true single-dose regimen exists: 1 gram orally as a single dose for adults and children >45 kg 2. However, this is for sexually transmitted infections, not prophylaxis for respiratory pathogens.