Azithromycin Tablet - Uses, Mechanism & Key Facts

What It Is

Azithromycin is a macrolide antibiotic (specifically a 15-atom azalide subclass) derived from erythromycin. It works by binding the 50S ribosomal subunit of bacteria, blocking protein synthesis. It is available as 250 mg and 500 mg tablets (as well as oral suspension and IV).

Katzung's Basic and Clinical Pharmacology, 16th Ed.
Lippincott Illustrated Reviews: Pharmacology

Approved Uses (Indications)

Respiratory Tract Infections

Condition	Typical Dosing
Community-acquired pneumonia (CAP)	500 mg Day 1, then 250 mg Days 2-5
Acute bacterial exacerbation of chronic bronchitis	500 mg/day x 3 days OR 500 mg Day 1 + 250 mg Days 2-5
Acute bacterial sinusitis (adults)	500 mg/day x 3 days
Pharyngitis / tonsillitis (alternative to penicillin)	500 mg Day 1, then 250 mg Days 2-5

Ear Infections

Acute otitis media (children ≥6 months): 30 mg/kg single dose OR 10 mg/kg/day x 3 days

Skin & Soft Tissue Infections

Uncomplicated skin and skin structure infections due to S. aureus, S. pyogenes, or S. agalactiae

Sexually Transmitted Infections (STIs)

Condition	Dosing
Chlamydial cervicitis / urethritis (C. trachomatis)	1 g single dose (alternative; doxycycline preferred)
Genital ulcer disease (chancroid, H. ducreyi)	1 g single dose
Mycoplasma urethritis	500 mg Day 1 + 250 mg Days 2-5

Other Infections

MAC (Mycobacterium avium complex) - prophylaxis and treatment in HIV patients
Toxoplasma gondii - has activity, used in combination regimens
Chlamydia pneumoniae / Mycoplasma pneumoniae - atypical pneumonia cover
Long-term/chronic use: Weekly azithromycin reduces exacerbations in bronchiectasis and COPD (intermittent macrolide therapy)

Why Azithromycin Has Unique Advantages

Long half-life (~68 hours) - allows once-daily dosing and short 3-5 day courses that still work
Excellent tissue penetration - concentrations in tissues are 10-100x higher than serum (especially in lungs, macrophages, neutrophils)
No CYP450 interactions - unlike erythromycin/clarithromycin, it does not inhibit cytochrome P450 enzymes, so fewer drug-drug interactions
Acid stable - absorbed well orally (though food can slightly delay absorption)
Primarily excreted in bile, not urine - no dose adjustment needed in renal impairment

Important Warnings & Precautions

QT prolongation: Azithromycin prolongs the QT interval (potassium channel effect). Avoid in patients with pre-existing QT prolongation, hypokalemia, hypomagnesemia, or those taking other QT-prolonging drugs
Cardiovascular risk: An FDA warning (2021) highlights the risk of potentially fatal cardiac arrhythmias
Antimicrobial resistance: The EMA has recommended restricting certain indications to minimize resistance development. Resistance occurs via efflux pumps, ribosomal methylation, or esterase production
Not appropriate for severe pneumonia requiring hospitalization or IV-only management

Common Side Effects

GI: nausea, diarrhea, abdominal pain (less than erythromycin)
Headache
Elevated liver enzymes (hepatotoxicity, rare)
Hearing loss (with high or prolonged doses)
Allergic reactions

Quick Dosing Summary (Adults)

Indication	Dose
Most infections	500 mg Day 1, 250 mg Days 2-5
STI (chlamydia/chancroid)	1 g single dose
CAP (severe, step-down)	500 mg IV x 2 days, then 500 mg oral to complete 7-10 days
MAC prophylaxis (HIV)	1,200 mg once weekly

Note: Azithromycin is NOT effective against MRSA, Pseudomonas, or most gram-negative enteric bacteria. It should always be prescribed based on culture and susceptibility when possible, to help preserve its effectiveness against macrolide-resistant organisms.

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