Azithromycin is a macrolide antibiotic commonly prescribed for bacterial sexually transmitted infections. In the UK, it is used to treat:
- Chlamydia – as a second-line option (doxycycline is now first-line per BASHH guidelines)
- Non-specific urethritis (NSU)
- Mycoplasma genitalium – often as part of a combination regimen
For chlamydia, the standard dose is 1 g as a single dose, sometimes followed by 500 mg daily for two additional days depending on clinical guidance.
Common Side Effects
Most people tolerate azithromycin well, but gastrointestinal side effects are relatively common. These include:
- Nausea – reported by up to 1 in 10 people
- Diarrhoea – usually mild and self-limiting
- Abdominal pain or cramping
- Vomiting – less common but possible, especially on an empty stomach
Taking azithromycin with food can help reduce stomach upset, although the single 1 g dose can be taken with or without food.
Less Common Side Effects
Some people may experience:
- Headache or dizziness
- Changes in taste
- Skin rash
- Fatigue
Rarely, azithromycin can cause QT prolongation — an alteration in heart rhythm. This is more relevant for people already taking other medications that affect the heart's electrical activity. If you have a known heart condition, your prescriber should be made aware.
Important Interactions
Inform your pharmacist or prescriber if you are taking:
- Antacids containing aluminium or magnesium — take azithromycin at least 1 hour before or 2 hours after
- Warfarin — azithromycin may enhance its anticoagulant effect
- Ergotamine — concurrent use is contraindicated
When to Seek Medical Advice
Contact your GP, pharmacist, or NHS 111 if you experience severe diarrhoea, signs of an allergic reaction (swelling, difficulty breathing, widespread rash), or palpitations after taking azithromycin. Most side effects are mild and resolve within a day or two.
Azithromycin remains an effective treatment option for certain STIs when prescribed appropriately. Always complete the course as directed and avoid alcohol in excess during treatment.