Mechanism of action
Azithromycin is a bacteriostatic macrolide antibiotic that inhibits bacterial protein synthesis. It binds reversibly to the 23S ribosomal RNA component of the 50S ribosomal subunit, blocking the translocation step of protein elongation. This prevents the movement of peptidyl-tRNA from the A site to the P site on the ribosome, halting bacterial growth.
As an azalide (a nitrogen-containing subclass of macrolides), azithromycin has enhanced acid stability and tissue penetration compared to erythromycin. It achieves high intracellular concentrations and is concentrated in phagocytes, which deliver the drug to sites of infection. The drug exhibits post-antibiotic effect, meaning bacterial suppression continues even after serum levels fall below the minimum inhibitory concentration.
Azithromycin is effective against gram-positive cocci (including Streptococcus pneumoniae and Streptococcus pyogenes), some gram-negative bacteria (Haemophilus influenzae, Moraxella catarrhalis), and atypical organisms (Mycoplasma pneumoniae, Chlamydia trachomatis, Legionella pneumophila). Resistance occurs through ribosomal methylation (erm genes), efflux pumps (mef genes), or ribosomal mutations.
- Azithromycin binds the 50S ribosomal subunit, inhibiting bacterial protein synthesis (bacteriostatic).
- Major adverse effect is QT prolongation with risk of torsades de pointes, especially in cardiac patients.
- Does NOT inhibit cytochrome P450 enzymes, unlike erythromycin and clarithromycin (fewer drug interactions).
- Long half-life (68 hours) allows once-daily dosing and 5-day courses; tissue levels persist for days.
- First-line for atypical pneumonia (Mycoplasma, Chlamydophila, Legionella) and chlamydial STIs.
- Safe in pregnancy (category B); preferred macrolide for pregnant women with chlamydial infections.
- Used for MAC prophylaxis in HIV/AIDS when CD4 <50 cells/mm³.
- Can cause infantile hypertrophic pyloric stenosis in neonates exposed in first weeks of life.
Indications
Azithromycin is used for a broad range of bacterial infections:
Respiratory tract infections
- Community-acquired pneumonia (including atypical pathogens)
- Acute bacterial exacerbations of chronic obstructive pulmonary disease
- Acute bacterial sinusitis
- Pharyngitis and tonsillitis (alternative to penicillin in penicillin-allergic patients)
Sexually transmitted infections
- Uncomplicated urethritis and cervicitis caused by Chlamydia trachomatis (single-dose regimen)
- Chancroid (Haemophilus ducreyi)
- Non-gonococcal urethritis
Skin and soft tissue infections
- Uncomplicated skin and skin structure infections
- Erysipelas and cellulitis (streptococcal)
Other indications
- Acute otitis media in children
- Prevention and treatment of disseminated Mycobacterium avium complex (MAC) in HIV/AIDS patients
- Pertussis (Bordetella pertussis)
- Prophylaxis for bacterial endocarditis in penicillin-allergic patients undergoing dental procedures
- Traveler's diarrhea (selected cases)
Azithromycin is often chosen for its convenient dosing regimen, excellent tissue penetration, and long half-life allowing once-daily or short-course therapy.
Pharmacokinetics
Absorption: Azithromycin is administered orally or intravenously. Oral bioavailability is approximately 37%, and absorption is reduced by food, so it should be taken on an empty stomach (1 hour before or 2 hours after meals) unless using certain formulations.
Distribution: The drug demonstrates extensive tissue distribution with a large volume of distribution (31 L/kg). Tissue concentrations exceed serum concentrations by 10- to 100-fold. Azithromycin concentrates in phagocytes, fibroblasts, and epithelial cells. It crosses the placenta and enters breast milk in small amounts.
Metabolism: Azithromycin undergoes minimal hepatic metabolism. It is not a substrate, inhibitor, or inducer of the cytochrome P450 system, which distinguishes it from erythromycin and clarithromycin and reduces drug-drug interaction risk via this pathway.
Elimination: The drug has a prolonged terminal elimination half-life of 68 hours, allowing once-daily dosing and extended activity after treatment completion. Approximately 50% is excreted unchanged in bile, and 6% is excreted unchanged in urine. No dosage adjustment is required for mild to moderate renal or hepatic impairment.
Special populations: Elderly patients show increased peak concentrations but similar overall exposure. Children have faster clearance and may require weight-based dosing.
Adverse effects
Azithromycin is generally well tolerated, but several important adverse effects must be recognized:
Gastrointestinal effects (most common)
- Diarrhea, nausea, abdominal pain, and vomiting occur in 5-10% of patients
- Usually mild and self-limiting
- Clostridioides difficile-associated diarrhea (antibiotic-associated colitis) can occur, though less common than with other antibiotics
Cardiovascular effects
- QT interval prolongation is a critical adverse effect; can precipitate torsades de pointes (polymorphic ventricular tachycardia)
- Risk is higher in patients with pre-existing QT prolongation, electrolyte abnormalities (hypokalemia, hypomagnesemia), structural heart disease, or concurrent use of other QT-prolonging drugs
- Fatal arrhythmias have been reported
Hepatotoxicity
- Transient elevations in liver enzymes (AST, ALT)
- Rare cases of cholestatic hepatitis, hepatic necrosis, and fulminant hepatic failure
- Discontinue if signs of hepatitis develop
Hypersensitivity reactions
- Rash, urticaria, angioedema
- Rare severe reactions: Stevens-Johnson syndrome, toxic epidermal necrolysis, acute generalized exanthematous pustulosis (AGEP)
Ototoxicity
- Reversible hearing loss, tinnitus, and vertigo (rare)
- More common with high doses or prolonged use
Other effects
- Headache, dizziness
- Photosensitivity (rare)
- Infantile hypertrophic pyloric stenosis (IHPS) in neonates exposed in first weeks of life
Contraindications and warnings
Contraindications:
- Hypersensitivity to azithromycin, erythromycin, or other macrolide/ketolide antibiotics
- History of cholestatic jaundice or hepatic dysfunction with prior azithromycin use
Warnings and precautions:
Cardiac risk
- Use with caution in patients with known QT prolongation, history of torsades de pointes, congenital long QT syndrome, bradycardia, uncompensated heart failure, or concurrent use of Class IA or III antiarrhythmics
- Correct electrolyte abnormalities before administration
- Consider ECG monitoring in high-risk patients
Hepatic impairment
- Use caution in patients with impaired hepatic function
- Monitor liver function in patients with baseline hepatic disease
Myasthenia gravis
- Macrolides may exacerbate muscle weakness; use with caution and monitor closely
Pregnancy and lactation
- Pregnancy category B (FDA classification): animal studies show no risk, but human data are limited
- Generally considered acceptable during pregnancy when clinically indicated
- Excreted in breast milk; use caution in nursing mothers
Resistance
- Overuse contributes to macrolide resistance; reserve for appropriate indications
- High resistance rates in Streptococcus pneumoniae and Streptococcus pyogenes in some regions
Superinfection
- Prolonged use may result in fungal or bacterial superinfection, including C. difficile infection
Drug interactions
Azithromycin has fewer drug interactions than erythromycin or clarithromycin because it does not significantly inhibit cytochrome P450 enzymes. However, important interactions exist:
QT-prolonging drugs
- Increased risk of arrhythmias when combined with other QT-prolonging agents:
- Class IA antiarrhythmics (quinidine, procainamide, disopyramide)
- Class III antiarrhythmics (amiodarone, sotalol, dofetilide)
- Fluoroquinolones (moxifloxacin, levofloxacin)
- Antipsychotics (haloperidol, ziprasidone, thioridazine)
- Tricyclic antidepressants
- Ondansetron
- Avoid concurrent use when possible; if necessary, monitor ECG
Antacids
- Aluminum- and magnesium-containing antacids reduce peak concentrations of azithromycin by up to 24%
- Separate administration by at least 2 hours
Warfarin
- Although azithromycin does not inhibit P450 enzymes, potentiation of anticoagulant effect has been reported
- Monitor INR when initiating or discontinuing azithromycin in patients on warfarin
Digoxin
- Macrolides may increase digoxin levels by altering gut flora that metabolize digoxin
- Monitor digoxin levels and signs of toxicity
Nelfinavir
- Azithromycin increases nelfinavir exposure; may require dose adjustment
- No significant interaction with most other antiretrovirals
Cyclosporine and tacrolimus
- Although weaker than other macrolides, azithromycin may increase immunosuppressant levels
- Monitor drug levels if used concurrently
Colchicine
- Potential for increased colchicine toxicity, especially in patients with renal or hepatic impairment
- Use lower colchicine doses or avoid combination
Clinical pearls
- Z-Pak dosing: The classic "Z-Pak" is a 5-day course with 500 mg on day 1, then 250 mg on days 2-5. Single-dose 1 g is used for uncomplicated chlamydial infections.
- Atypical pneumonia coverage: Azithromycin is a first-line agent for community-acquired pneumonia because it covers both typical and atypical pathogens (Mycoplasma, Chlamydophila, Legionella).
- Pregnancy safety: Unlike fluoroquinolones and tetracyclines, azithromycin is considered safe in pregnancy and is the drug of choice for chlamydial infections in pregnant women.
- Long tissue half-life: Therapeutic levels persist in tissues for 5-7 days after the last dose due to slow release from tissues, extending antimicrobial activity beyond the treatment course.
- Infantile hypertrophic pyloric stenosis: Use in neonates in the first weeks of life has been associated with IHPS; monitor infants for symptoms of gastric outlet obstruction.
- MAC prophylaxis in HIV: Azithromycin (weekly) or clarithromycin (daily) is used for primary prophylaxis of disseminated MAC in AIDS patients with CD4 counts <50 cells/mm³.
- Cardiac screening: In patients with cardiac risk factors, consider baseline ECG before administration and avoid combination with other QT-prolonging drugs.
- Resistance patterns: Macrolide resistance is increasing in S. pneumoniae and S. pyogenes; local resistance patterns should guide empiric therapy.
- No P450 interaction: Unlike erythromycin and clarithromycin, azithromycin does not inhibit CYP3A4, making it safer with drugs like statins, calcium channel blockers, and many others.
- Food interaction: Standard formulations should be taken 1 hour before or 2 hours after meals; extended-release formulations should be taken with food.
AZITHRO for azithromycin coverage: Atypicals (Mycoplasma, Chlamydia, Legionella), Z-pak dosing (5 days), Inhibits 50S ribosome, Traveler's diarrhea, Heart (QT prolongation risk), Respiratory infections, Outpatient STI treatment.
Practice questions
1. A 28-year-old woman presents with vaginal discharge and is diagnosed with uncomplicated cervicitis. A cervical swab confirms Chlamydia trachomatis infection. She has no known drug allergies and is not pregnant. Which of the following is the most appropriate treatment?
2. A 65-year-old man with a history of atrial fibrillation managed with amiodarone presents with community-acquired pneumonia. His ECG shows a QTc interval of 480 ms. Which of the following antibiotics should be avoided due to increased risk of cardiac arrhythmia?
3. A 32-year-old man with HIV (CD4 count 35 cells/mm³) presents for routine care. He is adherent to antiretroviral therapy but has never received prophylaxis for opportunistic infections. In addition to trimethoprim-sulfamethoxazole for Pneumocystis jirovecii, which prophylactic regimen should be initiated?
Frequently asked questions
How does azithromycin work?
Azithromycin works by binding to the 50S ribosomal subunit of bacteria, blocking protein synthesis and stopping bacterial growth. It is bacteriostatic, meaning it inhibits bacterial multiplication rather than directly killing bacteria. The drug achieves high intracellular concentrations and is particularly effective against atypical respiratory pathogens like Mycoplasma and Chlamydia, as well as common respiratory bacteria such as Streptococcus pneumoniae and Haemophilus influenzae.
What are the side effects of azithromycin?
The most common side effects of azithromycin are gastrointestinal, including diarrhea, nausea, abdominal pain, and vomiting. The most serious adverse effect is QT interval prolongation, which can lead to a life-threatening arrhythmia called torsades de pointes, especially in patients with heart disease or electrolyte imbalances. Other less common side effects include hepatotoxicity with elevated liver enzymes, allergic reactions ranging from rash to Stevens-Johnson syndrome, and reversible hearing loss with high doses.
Is azithromycin safe during pregnancy?
Yes, azithromycin is generally considered safe during pregnancy and is classified as FDA pregnancy category B. Animal studies have not shown harm to the fetus, though human data are limited. It is the preferred antibiotic for treating chlamydial infections in pregnant women and is commonly used for respiratory infections when antibiotics are needed during pregnancy. Unlike fluoroquinolones and tetracyclines, which are contraindicated in pregnancy, azithromycin has a favorable safety profile for both mother and fetus.
Azithromycin vs amoxicillin — what is the difference?
Azithromycin is a macrolide antibiotic that works by inhibiting bacterial protein synthesis, while amoxicillin is a beta-lactam antibiotic that kills bacteria by disrupting cell wall synthesis. Azithromycin covers atypical organisms like Mycoplasma and Chlamydia that amoxicillin does not, and has a longer half-life allowing once-daily dosing. Amoxicillin is more effective against Streptococcus pyogenes (strep throat) and is preferred for most streptococcal and many staphylococcal infections. Azithromycin does not inhibit cytochrome P450 enzymes and has fewer drug interactions than some antibiotics.
Can azithromycin cause heart problems?
Yes, azithromycin can cause QT interval prolongation on ECG, which increases the risk of a dangerous heart rhythm called torsades de pointes. This risk is highest in patients with pre-existing heart disease, electrolyte abnormalities (low potassium or magnesium), congenital long QT syndrome, or those taking other medications that prolong the QT interval. Fatal arrhythmias have been reported, though they are rare. Patients with significant cardiac risk factors should be screened with an ECG before receiving azithromycin, and electrolyte levels should be corrected if abnormal.
Conditions it is used for
Related drugs
Sources and further reading
DailyMed (FDA labels)StatPearls (NCBI Bookshelf)MedlinePlusWHO ATC/DDD index: J01FA10WHO ATC/DDD index: S01AA26
Educational content for medical students, prepared by the WhiteCoat Pro editorial team with AI assistance; every page is reviewed by a second AI model for factual errors. It is not medical advice and does not replace the product label, local guidelines or a clinician.