Azithromycin is a macrolide antibiotic, usually given as a short three- or five-day course. It treats chest, throat, ear and skin infections, and is a single-dose treatment for chlamydia and some other sexually transmitted infections. It is a common alternative for people allergic to penicillin.
What is Azithromycin?
Azithromycin is a macrolide antibiotic, dispensed as 250 mg and 500 mg tablets and as a paediatric suspension. It is a Schedule 4 prescription medicine, sold under its own name and as Azrotrin, Zithromax and Azimax.
It binds to the bacterial ribosome — the machinery a cell uses to build proteins — and stops the bacterium making them. Without protein synthesis it cannot grow or multiply, and the immune system clears the infection. Human ribosomes are structurally different, which is the basis of its selectivity.
Azithromycin’s defining feature is how long it stays in the tissues. It concentrates inside cells, particularly white blood cells, which carry it to sites of infection, and it is released slowly over days. Levels in infected tissue remain effective long after the last tablet.
That is why azithromycin is given as a three- or five-day course while most antibiotics run seven to fourteen days — and why a single 1 g dose treats chlamydia. The short course is not a lower-intensity treatment; it reflects genuinely different pharmacology.
Its practical importance in South Africa is as an alternative for people allergic to penicillin, since macrolides cover many of the same organisms through an unrelated mechanism, with no cross-reactivity.
What is Azithromycin commonly used for?
- Chest infections, including bronchitis and community-acquired pneumonia.
- Atypical pneumonia, caused by Mycoplasma, Chlamydophila and Legionella — organisms penicillins do not cover.
- Throat infections, including streptococcal tonsillitis in penicillin-allergic patients.
- Middle ear infections in children.
- Sinusitis.
- Skin and soft tissue infections.
- Chlamydia, as a single dose — one of its most common uses.
- Other sexually transmitted infections, including as part of treatment for gonorrhoea and chancroid.
- Travellers’ diarrhoea, where an antibiotic is warranted.
- Preventing infection in some people with HIV or chronic lung disease.
It does nothing for colds, flu or most coughs and sore throats, which are viral.
Two points worth being direct about.
For chlamydia, partners must be treated too. Chlamydia frequently causes no symptoms at all, which is why it spreads and why untreated infection can lead to pelvic inflammatory disease and infertility. Treating one person while their partner remains infected means reinfection within weeks. Avoid sex until both have completed treatment and any advised waiting period.
Azithromycin resistance is rising, partly because short courses are so convenient and it is used widely. That is a reason to take it when it is genuinely indicated and not otherwise.
How Azithromycin fits into this medicine category
| Group | Examples | Common targets |
|---|---|---|
| Macrolides | Azithromycin, erythromycin (Purmycin), clarithromycin | Chest, throat, ear, skin, STIs; penicillin allergy |
| Broad-spectrum penicillins | Amoxicillin | Chest, ear, throat, dental, urinary |
| Narrow-spectrum penicillins | Penicillin V (Betapen) | Strep throat |
| Quinolones | Ciprofloxacin (Cifran) | Urinary, gut, prostate |
| Nitroimidazoles | Metronidazole | Anaerobic and dental infections |
Azithromycin is considerably better tolerated than erythromycin, which causes marked stomach upset. See the antibiotics and antimicrobials hub.
What to check before using Azithromycin
Finish the course, even though it is short and even though you feel better — which you often will before the tablets run out.
Take it at the same time each day. It can be taken with or without food, though food reduces stomach upset.
Separate it from antacids containing aluminium or magnesium by at least two hours, as they reduce absorption.
Tell your prescriber or pharmacist if:
- You have a heart rhythm problem, particularly a long QT interval, or a family history of sudden cardiac death. Macrolides can prolong the QT interval.
- You have liver disease.
- You have kidney disease.
- You have myasthenia gravis, which macrolides can worsen.
- You are pregnant or breastfeeding.
- You take warfarin, digoxin, ciclosporin, a statin, or another medicine affecting heart rhythm.
- The medicine is for a child — the suspension is dosed by weight; shake it and use the measure supplied.
Follow the dispensing label. This page does not set doses, and the amount for a single-dose STI treatment differs entirely from a course for a chest infection.
Possible cautions and when to ask a pharmacist or doctor
The course being over does not mean the medicine has stopped working. Because azithromycin persists in tissue for days, the treatment continues after the last tablet. People sometimes worry that a three-day course cannot be enough — it is, for the infections it is prescribed for.
Stomach upset — nausea, abdominal pain, diarrhoea — is the commonest effect, though considerably milder than with erythromycin. Taking it with food helps.
Diarrhoea is frequent and usually mild. Severe, watery or bloody diarrhoea, particularly with fever and particularly if it begins after the course ends, can indicate Clostridioides difficile infection and needs medical attention.
Heart rhythm. Report palpitations or fainting, especially if you have an existing rhythm problem or take other QT-prolonging medicines.
Liver effects are uncommon but recognised. Report yellowing of the skin or eyes, dark urine or pale stools.
For an STI, attend any follow-up test offered, and complete partner notification. Clinics can help with this confidentially.
If there is no improvement in two or three days, or things worsen, go back.
Seek urgent medical attention for facial or throat swelling, difficulty breathing, a rash that blisters or peels, severe or bloody diarrhoea, palpitations or fainting, yellowing of the skin or eyes, or an infection spreading despite treatment.
Related medicine guides
Frequently asked questions
What is azithromycin used for? Chest, throat, ear and skin infections, and sexually transmitted infections including chlamydia. It is also used when someone is allergic to penicillin.
Why is the course only three days? Azithromycin concentrates in tissue and is released slowly, so it keeps working for days after the last tablet. The short course is by design.
Can I take it if I am allergic to penicillin? Yes — that is one of its main roles. It is a macrolide and works by a completely different mechanism, with no cross-reactivity.
Does a single dose really treat chlamydia? A single 1 g dose is an established treatment. Your partner must be treated too, and avoid sex until both of you have completed treatment.
Will it help my cold? No. Colds and flu are viral, and antibiotics have no effect on viruses.
Can I take it with an antacid? Separate them by at least two hours. Antacids containing aluminium or magnesium reduce absorption.