What is the recommended dosage and frequency of azithromycin (Zithromax) for treating bacterial infections?

Azithromycin Dosing for Bacterial Infections

The recommended azithromycin dosage for most bacterial infections in adults is 500 mg on day 1, followed by 250 mg once daily on days 2-5 (the standard "Z-pack"), while sexually transmitted infections typically require a single 1 gram dose, and pediatric dosing is weight-based at 10 mg/kg on day 1 followed by 5 mg/kg daily for 4 days. 1

Adult Dosing Regimens by Indication

Respiratory Tract Infections

• Standard 5-day regimen: 500 mg on day 1, then 250 mg once daily on days 2-5 for community-acquired pneumonia (mild severity), pharyngitis/tonsillitis, and uncomplicated skin/soft tissue infections 1
• Alternative 3-day regimen: 500 mg once daily for 3 consecutive days for acute bacterial exacerbations of COPD or acute bacterial sinusitis 1
• Both regimens provide the same total 1.5 gram dose and are equally effective for respiratory infections 2

Sexually Transmitted Infections

• Non-gonococcal urethritis/cervicitis (Chlamydia): Single 1 gram dose 1, 3
• Gonococcal infections: Single 2 gram dose, though azithromycin should NOT be used as monotherapy due to resistance—must combine with ceftriaxone 1, 2
• Persistent non-gonococcal urethritis: Use the extended 5-day regimen (500 mg day 1, then 250 mg for 4 days), not the single-dose regimen 3, 4

Specialized Indications

• Cat scratch disease: 500 mg on day 1, then 250 mg daily for 4 additional days (patients >45 kg) 5, 4
• Travelers' diarrhea/dysentery: Either single 1 gram dose or 500 mg daily for 3 days 3
• Babesiosis (with atovaquone): 500-1000 mg on day 1, then 250 mg once daily 4
• MAC prophylaxis in AIDS: 1,200 mg once weekly when CD4 <50 cells/μL 4

Pediatric Dosing Regimens

Standard Respiratory Infections

• 5-day regimen: 10 mg/kg (maximum 500 mg) on day 1, followed by 5 mg/kg once daily (maximum 250 mg/day) on days 2-5 for community-acquired pneumonia 5, 1
• 3-day regimen: 10 mg/kg once daily for 3 days for acute bacterial sinusitis 1
• Single-dose regimen: 30 mg/kg as a single dose (maximum 1500 mg) for acute otitis media 1

Pharyngitis/Tonsillitis

• 12 mg/kg once daily for 5 days (age ≥2 years) due to higher recurrence rates with standard dosing 1, 6

Pertussis

• Infants <6 months: 10 mg/kg per day for 5 days 4
• Children ≥6 months: 10 mg/kg (max 500 mg) on day 1, then 5 mg/kg (max 250 mg) on days 2-5 4

Cat Scratch Disease

• Patients <45 kg: 10 mg/kg on day 1, then 5 mg/kg for 4 additional days 5, 4

Critical Administration Considerations

Timing with Antacids

• Separate azithromycin from aluminum- or magnesium-containing antacids by at least 2 hours, as antacids reduce absorption by up to 50% 3, 2
• Azithromycin can be taken with or without food 1

Cardiac Monitoring

• Obtain baseline ECG in patients with cardiac risk factors before initiating therapy 2
• Avoid azithromycin if QTc >450 ms (men) or >470 ms (women) 2

Maximizing Adherence

• Dispense medication on-site and directly observe the first dose when possible, particularly for sexually transmitted infections 4, 2
• Single-dose regimens have the advantage of improved compliance and directly observed therapy 4

Common Pitfalls and Caveats

Resistance Concerns

• Azithromycin is NOT first-line for streptococcal pharyngitis due to increasing macrolide resistance in group A Streptococcus; use only as an alternative in penicillin-allergic patients 2
• Never use as monotherapy for gonococcal infections—must combine with ceftriaxone due to resistance 3, 2
• For Mycoplasma genitalium with macrolide resistance, switch to moxifloxacin 400 mg daily for 7-14 days 3

Dosing Errors

• Verify the mg/mL strength on the bottle label before calculating liquid doses—concentrations vary (100 mg/5 mL vs 200 mg/5 mL) 3
• For severe infections or immunocompromised patients, higher doses may be required (e.g., 600-1000 mg daily for babesiosis) 3

Special Populations

• Pregnancy: Azithromycin is FDA Pregnancy Category B and considered safe; preferred over erythromycin in infants <1 month due to lower risk of infantile hypertrophic pyloric stenosis 4, 2
• Renal impairment: No dosage adjustment needed for GFR ≥10 mL/min; exercise caution with severe impairment (GFR <10 mL/min) as AUC increases 35% 1
• Hepatic impairment: No established dosing recommendations; use with caution 1

Adverse Effects

• Most common: Gastrointestinal symptoms (abdominal discomfort, diarrhea, nausea, vomiting), headache, and dizziness—generally mild to moderate in severity 3, 4
• Better tolerated than erythromycin with fewer GI side effects 6, 7