What is the recommended dose of Azithromycin (macrolide antibiotic) for an adult patient with a bacterial infection, considering factors such as type of infection, weight, and underlying medical conditions like impaired renal (kidney) function or hepatic (liver) function?

Azithromycin Dosing for Adult Bacterial Infections

For most common bacterial infections in adults, azithromycin is dosed as 500 mg on day 1 followed by 250 mg once daily on days 2-5, or alternatively 500 mg once daily for 3 days, both delivering the same 1.5 gram total dose with equivalent efficacy. 1

Standard Adult Dosing by Infection Type

Respiratory Tract Infections

• Community-acquired pneumonia (mild): 500 mg on day 1, then 250 mg daily on days 2-5 1
• Acute bacterial exacerbations of COPD: Either 500 mg daily for 3 days OR 500 mg on day 1, then 250 mg daily on days 2-5 1
• Acute bacterial sinusitis: 500 mg once daily for 3 days 1
• Pharyngitis/tonsillitis (second-line): 500 mg on day 1, then 250 mg daily on days 2-5 1

The 3-day and 5-day regimens are equally effective because azithromycin's prolonged tissue half-life (68 hours) maintains therapeutic concentrations at infection sites for several days after administration ceases 2, 3. Tissue concentrations exceed serum levels by up to 100-fold and remain above MIC for relevant pathogens throughout the treatment course 3, 4.

Sexually Transmitted Infections

• Non-gonococcal urethritis/cervicitis (Chlamydia): Single 1 gram dose 1, 2
• Gonococcal urethritis/cervicitis: Single 2 gram dose 1
• Chancroid (genital ulcer disease): Single 1 gram dose 1

Critical caveat: Never use azithromycin as monotherapy for gonorrhea due to widespread resistance; always combine with ceftriaxone 2.

Skin and Soft Tissue Infections

• Uncomplicated skin/skin structure infections: 500 mg on day 1, then 250 mg daily on days 2-5 1

Gastrointestinal Infections

• Travelers' diarrhea/dysentery: Either single 1 gram dose OR 500 mg daily for 3 days 5
• Enteric fever (typhoid): 500 mg on day 1, then 250 mg daily on days 2-5 (total 5-7 days preferred to minimize relapse) 6

Special Populations and Dose Adjustments

Renal Impairment

No dose adjustment is required for patients with GFR 10-80 mL/min 1. However, exercise caution in severe renal impairment (GFR <10 mL/min), as AUC increases by 35% and Cmax by 61% 1, 5. In elderly patients, always assess renal function using estimated GFR rather than serum creatinine alone, as reduced muscle mass can produce falsely reassuring creatinine levels 5.

Hepatic Impairment

No specific dose adjustment recommendations exist for hepatic impairment, as pharmacokinetics have not been established in this population 1. Use with caution and increase monitoring if underlying liver disease is present 5.

Age and Weight Considerations

No dosage adjustment is recommended based on age or gender in adults 1. Standard adult dosing applies regardless of body weight for most indications.

Administration and Drug Interactions

• Can be taken with or without food 1, 5
• Avoid concurrent aluminum- or magnesium-containing antacids: Separate by at least 2 hours, as antacids reduce absorption by up to 50% 2, 5
• QT prolongation risk: Obtain baseline ECG before long-term therapy; contraindicated if QTc >450 ms (men) or >470 ms (women) 2

Long-Term Prophylactic Regimens

For specific chronic conditions requiring prolonged therapy:

• Bronchiectasis with ≥3 exacerbations/year: 500 mg three times weekly OR 250 mg daily for minimum 6 months 2, 5
• Disseminated MAC disease in AIDS: 250 mg daily with ethambutol ± rifabutin 2
• MAC prophylaxis in AIDS (CD4 <50): 1,200 mg once weekly 2

Before initiating long-term macrolide therapy: Obtain baseline liver function tests and ensure at least one negative respiratory NTM culture to minimize risk of NTM infection 2, 5.

Common Adverse Effects

Gastrointestinal symptoms (nausea, vomiting, abdominal pain, diarrhea) are the most common side effects, generally mild to moderate in severity and less frequent than with erythromycin 2, 5. Headache and dizziness may also occur 2.

Key Clinical Pitfalls to Avoid

1. Don't assume all liquid formulations have the same concentration: Verify mg/mL strength on bottle label before calculating dose 5
2. Don't use single-dose regimen for persistent infections: Extended 5-day regimen (500 mg day 1, then 250 mg for 4 days) is preferred for treatment failures 5
3. Don't use for severe pneumonia requiring ICU admission: Azithromycin is appropriate only for mild-moderate community-acquired pneumonia 1
4. Don't ignore macrolide resistance patterns: Verify local pneumococcal resistance rates before using for typical bacterial pneumonia 2