1. Clinical Overview
Azithromycin is a broad-spectrum macrolide antibiotic derived from erythromycin. It is characterized by an expanded spectrum of activity compared to older macrolides, excellent tissue penetration, and a prolonged half-life allowing for short-course dosing regimens (e.g., 3-day or 5-day courses). It is a first-line agent for many community-acquired infections in India due to its convenience, tolerability, and effectiveness against atypical pathogens.
| Onset | Duration | Bioavailability |
|---|---|---|
| Peak plasma concentrations are reached in 2-3 hours. Clinical symptom improvement is typically observed within 48-72 hours of initiating therapy. | Therapeutic tissue concentrations persist for 5-7 days after the last dose due to its long half-life and extensive tissue distribution. | Approximately 37% following oral administration, significantly influenced by food (reduced by 50% with a high-fat meal). |
2. Mechanism of Action
Azithromycin reversibly binds to the 50S ribosomal subunit of susceptible bacteria, specifically at the donor site (P site) of the peptidyl transferase center. This binding inhibits the translocation step of bacterial protein synthesis by preventing the transpeptidation/translocation reaction, leading to the arrest of peptide chain elongation and ultimately inhibiting bacterial growth (bacteriostatic effect).
3. Indications & Uses
- Acute bacterial exacerbations of chronic obstructive pulmonary disease (COPD)
- Community-acquired pneumonia (mild to moderate) caused by S. pneumoniae, H. influenzae, and atypical pathogens (M. pneumoniae, C. pneumoniae, L. pneumophila)
- Acute bacterial sinusitis
- Pharyngitis/Tonsillitis caused by S. pyogenes (as an alternative to first-line penicillin)
- Uncomplicated skin and skin structure infections (e.g., cellulitis, erysipelas, impetigo)
4. Dosage & Administration
Adult Dosage: For respiratory/skin infections: 500mg as a single dose on Day 1, followed by 250mg once daily on Days 2 through 5 (total dose 1.5g). Alternative 3-day regimen: 500mg once daily for 3 days. For sexually transmitted infections: Single 1g dose.
Administration: Administer at least 1 hour before or 2 hours after a meal for optimal absorption. The tablet should be swallowed whole with a full glass of water. Do not crush or chew. For the oral suspension, shake the bottle well before each use.
5. Side Effects
Common side effects may include:
- Nausea
- Abdominal pain/discomfort
- Diarrhea (can be watery)
- Flatulence
- Headache
- Dizziness
6. Drug Interactions
| Drug | Effect | Severity |
|---|---|---|
| Antacids (Aluminum/Magnesium hydroxide) | Reduces azithromycin absorption, decreasing serum levels. | Moderate |
| Warfarin | Potential increased anticoagulant effect and risk of bleeding; monitor INR closely. | Major |
| Digoxin | Azithromycin may increase digoxin bioavailability by altering gut flora, leading to toxicity. | Moderate |
| Cyclosporine, Tacrolimus | Potential increase in calcineurin inhibitor levels and risk of nephrotoxicity. | Major |
| Ergotamine/Dihydroergotamine | Increased risk of ergotism (severe peripheral vasospasm). | Major |
| Other QT-prolonging drugs (e.g., Fluoroquinolones, Amiodarone, Sotalol, Antipsychotics) | Additive risk of QT prolongation and life-threatening arrhythmias. | Major |
| Nelfinavir | Increases azithromycin serum concentration; monitor for increased side effects. | Moderate |
7. Patient Counselling
- DO take the medication exactly as prescribed, completing the full course even if you feel better.
- DO take the tablet on an empty stomach (1 hour before or 2 hours after food).
- DO inform your doctor about all other medications you are taking, including OTC drugs and supplements.
- DO NOT skip doses. If you miss a dose, take it as soon as you remember. If it's almost time for the next dose, skip the missed dose.
- DO NOT take antacids containing aluminum or magnesium within 2 hours of taking azithromycin.
8. Toxicology & Storage
Overdose: Symptoms are an extension of side effects: severe nausea, vomiting, diarrhea, temporary hearing loss, and profound lethargy.
Storage: Store at room temperature (15-30°C), protected from light and moisture. Keep in the original container, tightly closed. Keep out of reach of children. For oral suspension: After reconstitution, store at room temperature and use within 10 days. Do not freeze.