What it is
Azithromycin is a macrolide (chemically, an azalide) with its own pharmacological features: a very long half-life (over 60 hours) that allows short courses of 3 or 5 days, and good cover of atypical bacteria (Mycoplasma, Legionella, Chlamydia).
It is widely used in primary care because the course is convenient. It accumulates in tissues, especially the lung and macrophages. Its weak point in Spain is resistance: pneumococcus and the streptococcus that causes sore throats show significant macrolide resistance, so it is not a universal alternative to penicillin.
Mechanism of action
It works by inhibiting:
- Bacterial protein synthesis (it binds the 50S ribosomal subunit)
- It is especially active against Gram-positive cocci and atypical bacteria
- A bacteriostatic effect (it stops growth rather than killing directly)
At the counter
When to recommend it
- Atypical respiratory infections (Mycoplasma, Chlamydia)
- Exacerbations of COPD with a bacterial cause
- Bacterial pharyngitis or otitis, especially in people allergic to penicillin
- Mild to moderate community-acquired pneumonia
When not to
- Allergy to macrolides
- Heart disease with arrhythmias (risk of QT prolongation)
- Severe liver failure
- Patients taking other medicines that prolong the QT interval (some antiarrhythmics, antipsychotics, citalopram)
Warnings
- Important QT prolongation: risk of cardiac arrhythmias. Check the cardiac history and other medicines.
- Important Pseudomembranous colitis: very rare but serious. Warn about severe diarrhoea with blood.
- Caution Nausea and vomiting: quite common, especially at high doses or on an empty stomach.
- Caution Interactions: unlike clarithromycin and erythromycin, azithromycin barely inhibits CYP3A4. The main risk is adding it to other medicines that prolong the QT interval.
Pharmacokinetics
- Absorption
- Rapid oral absorption. Variable bioavailability of 30–50%. Capsules are best taken on an empty stomach; tablets can be taken with or without food. Peak concentration at 2–3 hours.
- Distribution
- EXCELLENT tissue distribution (lung, tonsils, macrophages). Lung concentrations above plasma levels. Protein binding of 50%. It crosses the placenta.
- Metabolism
- Little metabolism: most of it is eliminated unchanged. Its metabolites have no relevant activity.
- Elimination
- Mainly biliary. A remarkable half-life of 60–72 hours (which allows short courses). It accumulates in tissues.
Azithromycin vs co-amoxiclav
| Aspect | Azithromycin | Co-amoxiclav |
|---|---|---|
| Cover of atypical bacteria | Excellent (Mycoplasma, Chlamydia, Legionella) | Poor |
| Half-life | Very long (60 h) - only 5 days | Short (1.5 h) - 7–10 days |
| Lung penetration | Excellent | Good |
| Gram+ and anaerobe cover | Good but incomplete | Very good |
| Cardiac risk | Present (QT) | Minimal |
Azithromycin for atypical pneumonia, chronic respiratory infections, or when convenience matters (short course). Co-amoxiclav for the more common infections.
Self-assessment
Three questions. When you check your answers you will see the explanation for each one.
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Azithromycin is a macrolide with specific cover of atypical bacteria and a half-life so long that it allows short courses instead of the 7–10 days of others. Very useful in atypical pneumonia.
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Azithromycin can prolong the QT interval and cause arrhythmias in heart patients. It always needs a prior medical decision.
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A half-life of 60–72 hours allows azithromycin to build up in tissues. So taking it for only 5 days gives prolonged cover, because the drug persists.
Training content. It does not replace the summary of product characteristics or clinical judgement.