How the dose is calculated
Azithromycin's long tissue half-life allows once-daily dosing over a short course. This calculator uses the once-daily mg/kg dose for the selected indication; for the classic 5-day course (a higher day-1 loading dose followed by a lower days-2–5 maintenance dose), calculate the two phases separately using the two dedicated dropdown options rather than a single flat mg/kg/day figure across the whole course.
Typical dosing by indication
| Indication | Dose | Frequency | Max |
|---|---|---|---|
| Standard 3-day course | 10 mg/kg once daily | once daily × 3 days | 500 mg/day |
| 5-day regimen — Day 1 | 10 mg/kg once | once | 500 mg |
| 5-day regimen — Days 2–5 | 5 mg/kg once daily | once daily × 4 days | 250 mg/day |
| Typhoid fever | 10 mg/kg once daily | once daily × 7 days | 500 mg/day |
| Chlamydia trachomatis / NGU | 20 mg/kg (adult: 1 g flat) | single dose | 1000 mg |
| Legionnaires' pneumonia | 10 mg/kg once daily (oral or IV) | once daily × 2 weeks | 500 mg/day |
| Lymphogranuloma venereum (LGV) | 1 g flat (adult) | once weekly × 3 weeks | 1000 mg/dose |
| Donovanosis (granuloma inguinale) | 10 mg/kg once daily, or 1 g flat once weekly | × 7 days, or × 4 weeks (weekly) | 500 mg/day |
| Chancroid / PPNG urethritis | 20 mg/kg (adult: 1 g flat) | single dose | 1000 mg |
| Pertussis | Same as 5-day regimen above | once daily × 5 days | — |
The once-weekly regimens (lymphogranuloma venereum, and the weekly alternative for donovanosis) aren't something this daily mg/kg/day calculator can compute directly, since its frequency model is per-day, not per-week — for adults, use the flat 1 g dose directly once weekly for the stated number of weeks; for children, scale by weight up to the same 1 g cap, consistent with how the other single/flat-dose indications above are calculated.
Clinical use
- Azithromycin's long half-life and tissue penetration allow short courses (typically 3 days) or a 5-day regimen (higher loading dose on day 1, lower maintenance dose days 2–5) — check the specific product regimen being prescribed.
- Useful first-line option for atypical respiratory pathogens (Mycoplasma, Chlamydophila) and as a macrolide alternative in true penicillin allergy.
- Higher-dose, longer-course regimens are used for typhoid fever and some other specific indications — confirm the indication-specific course length, not just the daily dose.
- A single flat dose (1 g in adults, weight-based in children) is used for chlamydia trachomatis, non-gonococcal urethritis, and chancroid/PPNG urethritis rather than a multi-day course.
- Legionnaires' pneumonia can be treated with oral or IV azithromycin at the same once-daily mg/kg dose, typically for around 2 weeks (longer than the standard respiratory course) — severity and immune status guide the exact duration and oral-vs-IV choice.
- Lymphogranuloma venereum (LGV) and donovanosis (caused by Klebsiella granulomatis, formerly Calymmatobacterium granulomatis) are treated with a flat adult dose given weekly over several weeks — a genuinely different dosing pattern from the daily regimens used elsewhere on this page.
- Pertussis (whooping cough) is treated with the same day-1-loading/days-2–5-maintenance regimen used for the general 5-day course, ideally started early in the illness to reduce transmissibility (it has less effect on established cough once the paroxysmal phase is underway).
- Can prolong the QT interval — use caution with other QT-prolonging drugs, hypokalaemia/hypomagnesaemia, or known arrhythmia risk; this is a smaller effect than with erythromycin but is not absent.
- Causes clinically significant CYP3A4-mediated interactions less often than erythromycin or clarithromycin, but can still raise INR with warfarin and digoxin levels — review the patient's medication list regardless of which macrolide is chosen.
- Most common adverse effects are gastrointestinal (nausea, diarrhoea, abdominal pain) — generally milder than with erythromycin. Rare but serious effects include hepatotoxicity, hypersensitivity reactions, and (as above) QT prolongation.
- Considered one of the macrolides with the most reassuring safety data in pregnancy when a macrolide is indicated, though it should still only be used when clearly needed, per usual pregnancy-prescribing principles.
Administration
The oral suspension is generally best taken on an empty stomach (1 hour before or 2 hours after food) for more consistent absorption, though the effect of food on overall exposure is modest compared with some other antibiotics. Some tablet formulations may be taken with or without food — check the specific product's label. Shake the suspension well before each dose, and use an oral dosing syringe rather than a household spoon for accuracy.
Frequently asked questions
What is the azithromycin dose for typhoid fever?
10 mg/kg once daily for 7 days, up to a maximum of 500 mg/day — select "Typhoid fever" above and enter the patient's weight for the exact per-dose amount.
What is the azithromycin dose for Legionnaires' disease/pneumonia?
10 mg/kg once daily (oral or IV) for about 2 weeks, up to a maximum of 500 mg/day — the same once-daily mg/kg dose is used whether given orally or intravenously.
What is the azithromycin dose for lymphogranuloma venereum (LGV)?
1 g (flat adult dose) once weekly for 3 weeks. This weekly schedule falls outside what this daily-dosing calculator computes directly — for adults, give the 1 g dose once a week for 3 weeks; for children, scale by weight up to the same 1 g cap.
What is the azithromycin dose for donovanosis (granuloma inguinale)?
Either 500 mg once daily for 7 days, or 1 g once weekly for 4 weeks — both are recognised regimens for donovanosis (caused by Klebsiella granulomatis). The daily option can be calculated above; the weekly option uses the same flat 1 g adult dose repeated weekly.
What is the azithromycin dose for chancroid?
1 g as a single dose (weight-based in children, up to the same 1 g adult cap) — the same single-dose regimen used for PPNG (penicillinase-producing Neisseria gonorrhoeae) urethritis.
Why is azithromycin often a 3-day course?
Its long tissue half-life means drug concentrations stay therapeutic for several days after the last dose, so a short 3-day course is often as effective as a longer course of drugs with a shorter half-life.
What is the 5-day azithromycin regimen?
Some products use a higher loading dose on day 1 (about 10 mg/kg) followed by a lower maintenance dose (about 5 mg/kg) on days 2–5 — check which regimen your product/guideline specifies rather than assuming a flat daily dose throughout.
Is azithromycin safe in penicillin allergy?
Yes — azithromycin is a macrolide, structurally unrelated to penicillins, and is commonly used as an alternative in patients with true penicillin allergy for suitable indications.
Does azithromycin interact with other medications?
Yes — notably other QT-prolonging drugs, and it can increase levels of some drugs metabolised by the same pathways. Review the patient's full medication list before prescribing.
What are the most common side effects of azithromycin?
Gastrointestinal upset — nausea, diarrhoea, and abdominal pain — is the most common, and is generally milder than with older macrolides like erythromycin. Serious effects (hepatotoxicity, significant hypersensitivity reactions, QT prolongation) are uncommon but worth being alert to.
Should azithromycin be taken with or without food?
The suspension is generally best given on an empty stomach for more consistent absorption, though the effect of food is modest. Check the specific product label, since some tablet formulations can be taken with or without food.
Is azithromycin safe in pregnancy?
It's generally considered one of the macrolides with more reassuring safety data in pregnancy when a macrolide antibiotic is clinically indicated, but as with any medication in pregnancy, it should only be used when the benefit is judged to outweigh the risk, and ideally after discussion with the treating obstetric team.
How is azithromycin different from erythromycin and clarithromycin?
All three are macrolides, but azithromycin has a much longer tissue half-life (allowing shorter courses and once-daily dosing), tends to cause less gastrointestinal upset than erythromycin, and has fewer clinically significant CYP3A4 drug interactions than clarithromycin or erythromycin — though it is not interaction-free.
References
- World Health Organization. Guidelines for the Management of Typhoid Fever.
- Bradley JS, et al. PIDS/IDSA Community-Acquired Pneumonia Guideline. Clin Infect Dis. 2011.
- British National Formulary for Children (BNFc) — Azithromycin.
- CDC. Sexually Transmitted Infections Treatment Guidelines — Chlamydial Infections.
- CDC. Pertussis (Whooping Cough) — Treatment and Antimicrobial Prophylaxis.
- Tripathi KD. Essentials of Medical Pharmacology, 8th edition — pages 762, 804, 813.