azithromycin

macrolide antibiotic· Brands: Zithromax, Z-Pak, Azithrocin, Azyth, ZmaxIVPO

Mechanism of action

Azithromycin binds to the 50S subunit of the bacterial ribosome, inhibiting protein synthesis. It is bacteriostatic but can be bactericidal at high concentrations. It has activity against many Gram-positive, Gram-negative, and atypical pathogens.

Pharmacokinetics

Azithromycin is well absorbed orally, with an elimination half-life of about 68 hours. It has extensive tissue distribution and high intracellular concentrations. It is metabolized in the liver and eliminated mainly in bile and faeces. Food may affect absorption of capsules, but not the suspension.

Indications & dosing

Community-acquired pneumonia

For 3 days; alternative regimen: 500 mg on day 1 then 250 mg days 2-5.

adult · PO: 500 mg once daily · For 3 days; alternative regimen: 500 mg on day 1 then 250 mg days 2-5.

Acute otitis media

Single dose of 30 mg/kg (max 1500 mg); alternative: 10 mg/kg once daily for 3 days.

pediatric · PO: 30 mg/kg single dose · Single dose of 30 mg/kg (max 1500 mg); alternative: 10 mg/kg once daily for 3 days.

Chlamydial urethritis/cervicitis

For uncomplicated genital chlamydia; give as single oral dose.

adult · PO: 1000 mg single dose · For uncomplicated genital chlamydia; give as single oral dose.

Streptococcal pharyngitis (penicillin allergy)

For 3 days in penicillin-allergic patients.

adult · PO: 500 mg once daily · For 3 days in penicillin-allergic patients.

Skin and soft tissue infections

For 3 days; alternative: 500 mg on day 1 then 250 mg days 2-5.

adult · PO: 500 mg once daily · For 3 days; alternative: 500 mg on day 1 then 250 mg days 2-5.

Contraindications & precautions

Hypersensitivity to macrolide antibiotics · Includes azithromycin, erythromycin, clarithromycin.
Previous cholestatic jaundice or hepatic dysfunction associated with macrolide use · Can be a sign of macrolide-induced liver injury.
Known QT interval prolongation or risk factors for torsades de pointes · Avoid in prolonged QT, hypokalaemia, or concomitant QT-prolonging drugs.
Severe hepatic impairment · Use with caution; monitor liver function.
Take with food or immediately after a meal to reduce gastrointestinal discomfort.
Monitor for signs of cardiac arrhythmias, especially with concurrent QT-prolonging drugs or in patients with electrolyte imbalance.
Use with caution in patients with myasthenia gravis; may exacerbate muscle weakness.
Caution in patients with hepatic impairment; advise reporting symptoms of liver dysfunction.
Consider drug interactions with antacids (aluminium/magnesium), warfarin, digoxin, and statins; manage accordingly.

Adverse effects

Diarrhoea
Nausea
Abdominal pain
Headache
QT prolongationserious
Clostridioides difficile-associated diarrhoeaserious
Hepatotoxicityserious
Severe cutaneous adverse reactions (SJS/TEN)serious
Anaphylaxis/angioedemaserious

Interactions

amiodaronemajorAvoid concurrent use if possible. If co-administration is necessary, use the lowest effective doses and closely monitor the QT interval with electrocardiography before and during therapy. Assess and correct electrolyte imbalances. Consider alternative antibiotics that do not prolong the QT interval. Seek specialist advice if the patient has additional risk factors.
warfarinmajorIf co-administration is necessary, monitor INR frequently (e.g., within 3-5 days of starting or stopping azithromycin) and adjust warfarin dose as needed. Consider alternative antibiotics with lower interaction potential. Advise patients to report signs of bleeding.

Renal & hepatic adjustment

eGFR 10+: No dose adjustment required
eGFR 0–10: Use with caution; no specific dose adjustment defined
Hepatic impairment (mild): No dose adjustment required
Hepatic impairment (moderate): No dose adjustment required
Hepatic impairment (severe): Use with caution; monitor for hepatotoxicity

Monitoring, pregnancy & toxicity

Liver function tests · Baseline and if symptoms of hepatic dysfunction develop
ECG · In patients at risk of QT prolongation
Signs of hypersensitivity · During treatment
INR · If concurrent warfarin use
Pregnancy: Azithromycin is generally considered safe for use during pregnancy when clinically indicated. It may be used for infections such as chlamydia in pregnancy.
Toxicity: Gastrointestinal upset (nausea, vomiting, diarrhoea) · Supportive care; ensure hydration
Toxicity: QT prolongation · Stop drug and monitor ECG; correct electrolyte abnormalities
Toxicity: Hepatotoxicity · Discontinue therapy and monitor liver function
Toxicity: Ototoxicity (reversible hearing loss) · Usually reversible; discontinue if severe

Clinical notes

Azithromycin is used for respiratory tract infections, skin infections, and sexually transmitted infections, including chlamydia and gonorrhoea. Caution is needed in patients with QT prolongation or taking other drugs that prolong QT. Doses vary by indication and region; follow local guidelines. Common adverse effects include gastrointestinal upset, headache, and rash.

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BNF; local formulary