How the dose is calculated
Metronidazole dosing depends heavily on the indication — anaerobic bacterial infection, invasive amoebiasis, and giardiasis all use different mg/kg/day targets and course lengths. Select the indication being treated to get the correct dose and frequency.
Typical dosing by indication
| Indication | Dose | Frequency | Max |
|---|---|---|---|
| Anaerobic infection | 30 mg/kg/day | every 8h (3×/day) | 1500 mg/day |
| Amoebiasis | 40 mg/kg/day | every 8h (3×/day) | 2250 mg/day |
| Giardiasis | 15 mg/kg/day | every 8h (3×/day) | 750 mg/day |
These mg/kg/day targets are the same whether you're working out the metronidazole dose for adults or the metronidazole dose for a child — there's no separate flat adult dose the way some other drugs have. Only the weight entered changes; enter the patient's weight (adult or paediatric) and the calculator applies the same per-kg target and dose caps either way.
Injection (IV) metronidazole / Metrogyl dosing
When a patient can't take oral or nasogastric metronidazole, the same drug is given as an intravenous infusion — commonly under the brand name Metrogyl (metronidazole is also sold as Flagyl and other brands). Inj. metronidazole/Metrogyl is usually supplied ready-mixed as a 500 mg / 100 mL infusion bag (5 mg/mL) and given by slow IV infusion over 20–60 minutes, at the same mg/kg/day target as the oral dose above — switch back to oral once the patient can tolerate it (IV-to-oral switch therapy).
| Weight | Per-dose (anaerobic infection, 30 mg/kg/day) | Inj. Metrogyl/metronidazole IV volume (500 mg/100 mL) |
|---|---|---|
| 5 kg | 50 mg | 10 mL |
| 10 kg | 100 mg | 20 mL |
| 15 kg | 150 mg | 30 mL |
| 20 kg | 200 mg | 40 mL |
| 25 kg | 250 mg | 50 mL |
| 30 kg | 300 mg | 60 mL |
| 40 kg | 400 mg | 80 mL |
| 50 kg+ / adult | 500 mg (capped) | 100 mL (one standard bag) |
This IV reference uses the anaerobic-infection target (30 mg/kg/day); use the calculator above with the correct indication selected for amoebiasis or giardiasis dosing instead, and apply the same per-dose mg figure to whatever concentration of injection is stocked locally.
Clinical use
- Metronidazole is a key agent against anaerobic bacteria and protozoal infections (amoebiasis, giardiasis, trichomoniasis) — the correct indication must be selected since dosing and course length differ substantially between them.
- Course length varies by indication — anaerobic bacterial infections are typically 7–10 days, invasive amoebiasis 5–10 days, and giardiasis 5–7 days; confirm the specific course length against current guidance.
- Avoid alcohol during treatment and for at least 48 hours after the last dose — a disulfiram-like reaction (flushing, nausea, tachycardia) can occur.
- Metallic taste, nausea, and dark urine are common but harmless side effects worth counselling patients about in advance.
Frequently asked questions
Is Metrogyl the same as metronidazole?
Yes — Metrogyl is simply one of the common brand names for metronidazole (others include Flagyl), available as tablets, oral suspension, and IV infusion. They're the same medicine, so this calculator's doses apply to Metrogyl or any other metronidazole brand equally.
What is the metronidazole dose per kg?
It depends on the indication: 30 mg/kg/day for anaerobic bacterial infection, 40 mg/kg/day for invasive amoebiasis, and 15 mg/kg/day for giardiasis — each split into 3 doses a day (every 8 hours), up to the maximum daily dose for that indication. Select the indication above and enter the weight to get the exact per-dose mg per kg.
What is the injection (IV) metronidazole / Metrogyl dose?
The IV dose uses the same mg/kg/day target as the oral dose for the same indication — it isn't a separate, lower or higher number. Inj. metronidazole/Metrogyl is typically supplied ready-mixed as a 500 mg/100 mL infusion bag and given by slow IV infusion every 8 hours; see the injection dosing table above for the per-dose mg and mL by weight.
What is the paediatric dose of metronidazole?
Like the adult dose, the paediatric metronidazole dose is calculated in mg/kg/day and depends on the indication being treated — this calculator uses the same weight-based mg/kg/day targets for children as for adults, up to the same maximum daily dose caps, so entering a child's weight and selecting the indication gives the correct paediatric dose and suspension volume.
Why does metronidazole dosing differ by indication?
Anaerobic bacterial infections, invasive amoebiasis, and giardiasis each need a different mg/kg/day target and course length to reliably clear the specific organism, so the indication must be selected correctly rather than using one flat dose for everything.
Can I drink alcohol while taking metronidazole?
No — alcohol should be avoided during treatment and for at least 48 hours after the last dose, as it can cause a disulfiram-like reaction with flushing, nausea, and a rapid heartbeat.
Is a metallic taste normal with metronidazole?
Yes — a metallic taste, along with nausea or darker urine, is a common and harmless side effect of metronidazole that resolves after finishing the course.
What is metronidazole used for in giardiasis?
It's one of the standard treatments for Giardia lamblia infection, dosed by weight for a shorter course (typically 5–7 days) than the doses used for bacterial or amoebic infections.
References
- World Health Organization. Model Formulary — Metronidazole.
- British National Formulary for Children (BNFc) — Metronidazole.
- Centers for Disease Control and Prevention. Giardiasis — Treatment Guidance.