Metronidazole: veterinary dose and monograph

At a glance

Drug class

Gastrointestinal

Also known as

Flagyl

Species with doses

Dogs, Cats, Horses, Birds, Rabbits, Ferrets, Guinea pigs, Reptiles, Chinchillas, Hamsters, Rats, Mice, Gerbils, Sugar gliders and Primates

General dose range

10–25 mg/kg PO q12h — check the species tables for the indication

Routes

PO, IV

Last reviewed

1 October 2026

Indications

• Giardia (dogs, cats, other species)
• Anaerobic infections
• Inflammatory bowel disease (large bowel)
• Clostridium perfringens enterotoxicosis
• H. pylori eradication (combination therapy)
• Hepatic encephalopathy (cats)
• Foal Clostridium diarrhoea

Metronidazole dose by species

Doses are per administration unless stated. mg/kg doses scale with body weight; per-animal, per-quarter and infusion-rate doses are shown as the reference writes them.

Dogs

• Giardia: Plumb’s p.827 (Lappin 2006): 15–25 mg/kg PO q12–24h for 5–7 days; 22 mg/kg twice daily for 5 days, optionally with fenbendazole 50 mg/kg daily (Payne). Papich 5e: 12–15 mg/kg q12h for 5–7 days. Doses are for metronidazole base (benzoate is 62% base).
• Anaerobic sepsis: Plumb’s p.827 (Hardie 2000): 15 mg/kg IV q12h. Papich 5e: 15 mg/kg IV as a slow diluted infusion over at least 30 minutes to avoid CNS effects.
• IBD / colitis: Plumb’s p.827: large-bowel IBD 10–20 mg/kg PO two to three times daily (Washabau); refractory colitis 10–20 mg/kg q8–12h for 2–4 weeks (Leib); lymphocytic–plasmacytic enteritis 10–30 mg/kg q8–24h. Watch for neurotoxicity on long courses.
• Anaerobic infection (oral): Papich 5e: anaerobic bacteria 15 mg/kg q12h or 12 mg/kg q8h PO. IV: 15 mg/kg as a slow diluted infusion over at least 30 minutes.

Cats

• Giardia: Plumb’s p.827: 15–25 mg/kg PO q12–24h for 5–7 days (Lappin); 25 mg/kg q12h for 7 days (Zoran). Papich 5e: 17 mg/kg (⅓ tablet) q24h for 8 days, or metronidazole benzoate 25 mg/kg q12h for 7 days (benzoate is 62% base).
• Hepatic encephalopathy: Plumb’s p.827 (Cornelius et al. 2000): 7.5 mg/kg PO q8–12h — use the lower dose because hepatic clearance is reduced.
• Anaerobic infection: Papich 5e: cats, anaerobic bacteria 10–25 mg/kg q24h PO. Metronidazole benzoate is 62% metronidazole — adjust if using the benzoate salt.

Horses

• Susceptible anaerobic infection: Plumb’s p.827: 20–25 mg/kg PO q8–12h (rectally at the same dose if oral dosing is impossible), Clostridial colitis 15 mg/kg PO q8h (Bentz); B. fragilis metritis 15–25 mg/kg PO q12h (LeBlanc). Papich 5e: 10 mg/kg q12h PO — higher doses (up to 15–20 mg/kg q6h) carry more side effects.
• Foal Clostridial diarrhoea: Plumb’s p.827: foals 15–25 mg/kg PO q8h (25 mg/kg q12h also recommended); IV loading 15 mg/kg then 7.5 mg/kg. Papich 5e: foals 10–15 mg/kg IV or PO q12h.

Birds

• Anaerobes / Giardia: Plumb’s p.828 (Oglesbee 2009): 10–50 mg/kg PO q12h for anaerobes and Giardia. Ratites (not for food): 20–25 mg/kg PO twice daily (Jenson 1998).

Rabbits

• Anaerobic infection: Plumb's p.828 (Ivey & Morrisey 2000): rabbits 20 mg/kg PO q12h ×3–5 days, or 40 mg/kg PO q24h; 5 mg/kg slow IV q12h.

Ferrets

• Anaerobic / Giardia; Helicobacter (triple therapy): Plumb's p.828 (Williams 2000): 10–30 mg/kg PO once–twice daily; very bitter — mask flavour. Giardia 50 mg/kg PO q24h. H. mustelae triple therapy: metronidazole 20–22 mg/kg with amoxicillin + bismuth subsalicylate ×21–28 days.

Guinea pigs

• Anaerobic / antiprotozoal: Plumb's p.828: Chinchillas/Gerbils/Guinea Pigs/Hamsters/Mice/Rats 20–60 mg/kg PO q8–12h; guinea pigs also 10–40 mg/kg PO q24h.

Reptiles

• Anaerobic infection: Plumb's p.828: most reptile species 150 mg/kg PO once, repeat in one week. For flagellate/Giardia protozoa lower repeated doses are used per species reference.
• Amoebae / flagellates (antiprotozoal): Plumb’s p.828 (Gauvin 1993): 100–275 mg/kg PO once, repeat in 1–2 weeks. SPECIES WARNING: indigo snakes (Drymarchon) and tricolour kingsnakes (Lampropeltis pyromelana, L. zonata) — 40 mg/kg PO once, repeat in 2 weeks. Alternative (de la Navarre 2003): 50 mg/kg PO q24h for 3–5 days.

Chinchillas

• Antimicrobial / Giardia: Plumb's p.828 (Hayes 2000): 10–40 mg/kg PO q24h as antimicrobial; 50–60 mg/kg PO q12h ×5 days as antiparasitic (Giardia).

Hamsters

• Anaerobic / antiprotozoal: Plumb's p.828: 20–60 mg/kg PO q8–12h; gerbils/hamsters dosed at 7.5 mg per 70–90 g body weight PO q8h.

Rats

• Anaerobic / antiprotozoal: Plumb's p.828: 20–60 mg/kg PO q8–12h; also dosed 10–40 mg per rat PO q24h.

Mice

• Anaerobic / antiprotozoal: Plumb's p.828: 20–60 mg/kg PO q8–12h; or 3.5 mg/mL in drinking water for 5 days.

Gerbils

• Anaerobic / antiprotozoal: Plumb's p.828: 20–60 mg/kg PO q8–12h; gerbils/hamsters 7.5 mg per 70–90 g PO q8h. Add sucrose to improve palatability.

Sugar gliders

• Anaerobic / protozoal infection: Carpenter Exotic Animal Formulary 6th ed, Table 7-1 (p.497): sugar gliders 25 mg/kg PO q12–24h ×7–10 days. CNS toxicity possible at high doses / with hepatic disease; compound the benzoate form (better palatability).

Primates

• Anaerobic / protozoal infection (prosimians): Carpenter Exotic Animal Formulary 6th ed, Table 13-1 (p.683): prosimians 25 mg/kg PO q24h; chimpanzees 25 mg/kg PO q12h; monkeys/tamarins 15 mg/kg IV loading then 7.5 mg/kg IV q8h (do not exceed 4 g/day).

Contraindications

PROHIBITED in food animals (FDA). Hypersensitivity. Avoid in severely debilitated, pregnant, or nursing animals. Reduce dose by 25–50% in significant hepatic impairment. Total daily dose in dogs should NOT exceed 65 mg/kg/day (neurotoxicity).

Species-specific warnings

Dogs: Do NOT exceed 65 mg/kg/day total. Neurotoxicity (vestibular ataxia, seizures) is dose-dependent and cumulative. Monitor closely beyond 14 days.

Cats: Cats are more sensitive to metronidazole neurotoxicity than dogs. Use lower end of dose range. Genotoxic effects reported (transient).

Birds: Use lower doses (10–20 mg/kg) for small psittacines. Higher doses (>30 mg/kg) risk neurotoxicity in small birds.

Rabbits: AVOID — metronidazole can cause fatal dysbiosis / enterotoxaemia in rabbits.

Cattle: PROHIBITED in food-producing animals (FDA). Do NOT use in cattle, dairy cows, or veal calves.

Adverse effects

• Neurotoxicity (ataxia, seizures, vestibular signs) — acute high dose or chronic moderate-high dose
• Hepatotoxicity
• GI: anorexia, vomiting, diarrhoea
• Neutropenia, hepatotoxicity, haematuria (dogs)
• Genotoxicity (cats, transient)

Drug interactions

• Phenobarbital / phenytoin: increased hepatic metabolism of metronidazole — may need higher dose
• Cimetidine: decreased metronidazole clearance — risk of toxicity
• Warfarin / oral anticoagulants: potentiates anticoagulant effect — monitor PT/INR
• Cyclosporine: metronidazole may increase cyclosporine levels
• Alcohol / ethanol-containing products: disulfiram-like reaction (N/V, abdominal cramps)
• Fluoroquinolones: synergistic against mixed aerobic-anaerobic infections

Pregnancy and lactation

AVOID in pregnant animals — teratogenic / mutagenic in laboratory animals. Crosses the placenta rapidly. Excreted in milk — avoid in lactating dams or allow 24–48h withdrawal before nursing resumes. FDA Pregnancy Category B (human) but veterinary consensus is to avoid in breeding/pregnant animals.

Monitoring

• Baseline and periodic CBC — monitor for neutropenia (rare)
• Hepatic enzymes (ALT, ALP) before and during prolonged courses
• Neurological assessment — watch for vestibular signs, ataxia, seizures
• Daily dose calculation — do NOT exceed 65 mg/kg/day in dogs
• Faecal examination (Giardia cases) — repeat at end of treatment to confirm clearance

Toxicity and overdose

Dogs: LD50 not precisely established; neurotoxicity seen at >60 mg/kg/day chronically or acute doses >100 mg/kg. Cats: more sensitive — neurotoxicity reported at lower cumulative doses. Signs: vestibular ataxia, head tilt, nystagmus, seizures, opisthotonus. Onset usually 7–12 days into therapy but can occur acutely with overdose.

Management: No specific antidote. Discontinue immediately — most signs resolve in 1–2 weeks. Supportive: IV fluids, anti-seizure therapy (diazepam 0.5 mg/kg IV prn for seizures), anti-nausea (maropitant). GI decontamination (activated charcoal) if within 2h of ingestion. Diazepam has been reported to accelerate neurological recovery in dogs.

Clinical pearls

• Sharp metallic taste — conceal in empty gelatin capsule or compound into flavoured suspension for cats
• Giardia: combine with fenbendazole (50 mg/kg PO q24h x 5 days) for higher cure rates
• IBD: often used as part of a "metronidazole + prednisolone" protocol for chronic large-bowel colitis
• If neurotoxicity develops, drug effect is self-limiting — expect 1–2 week recovery after discontinuation
• Benzoate formulation exists for cats that refuse tablets — 25 mg/mL suspension

Mechanism of action

Nitroimidazole pro-drug. The nitro group is reduced by anaerobic organisms’ electron transport proteins to form cytotoxic short-lived intermediates that bind to DNA, causing strand breakage and cell death. Activity is strictly limited to anaerobic bacteria and protozoa (Giardia, Trichomonas, Entamoeba). Also has immunomodulatory effects — suppresses cell-mediated immunity, which contributes to its efficacy in IBD.

Formulations and products

• Flagyl 200 mg tab (Sanofi)
• Flagyl 400 mg tab (Sanofi)
• Flagyl 500 mg tab
• Metrogyl 200 mg / 400 mg tab (JB Chemicals)
• Flagyl / Metrogyl oral suspension 25 mg/mL (200 mg/4 mL benzoate)
• Flagyl IV / Metrogyl IV infusion 5 mg/mL (100 mL bag)
• Cflex-Tz IU intrauterine 30 mg/mL — cipro 25 + tinid 30 (Intas)
• Cflex-Tz bolus 1.8 g tinidazole (Intas)
• Lenovo-AP IU 40 mg/mL ornidazole (Intas)
• Metrogyl Vet (J.V. Pharma) — 200 mg, Tablet
• Metrocare Vet (Vetoquinol) — 250 mg, Tablet
• Metrozol Vet (Intas) — 200 mg, Tablet
• Cflex-Tz (Intas (Neovet)) — Cipro 1.5 g + Tinidazole 1.8 g, Large-animal oral bolus
• Cflex-Tz Tablets (Intas (Neovet)) — Cipro 250 mg + Tinidazole 300 mg, Small-animal tablet
• Cflex-Tz I.U. (Intas (Neovet)) — Cipro 125 + Tinid 150 / 5 mL, Intrauterine suspension
• Lenovo-AP (Intas (Neovet)) — Levofloxacin 20 + Ornidazole 40 mg/mL + Vit E, Intrauterine solution
• Oflokind-M Pet Suspension (Pet Mankind (Mankind Pharma)) — Ofloxacin 50 mg + Metronidazole Benzoate 100 mg per 5 mL, Mango-flavoured oral suspension
• Flagyl — 200 mg / 400 mg, Tablet (human-label)
• Metrogyl — 200 mg / 400 mg, Tablet (human-label)
• Flagyl Suspension — 200 mg / 5 mL (40 mg/mL), Oral suspension (human-label)
• Metrogyl Suspension — 100 mg / 5 mL benzoate, Oral suspension (human-label)
• Flagyl IV — 5 mg/mL, IV infusion (human-label)
• Metrogyl IV — 5 mg/mL, IV infusion (human-label)

Storage

Store tablets at 15–30°C (59–86°F) protected from light. IV solution: do not refrigerate — crystals may form. Reconstituted oral suspensions stable for 14 days at room temp.

Before you use this dose

Reference doses for veterinary professionals. Confirm the dose against the product label, the patient’s condition, current guidance and local regulations before use; the attending veterinarian remains responsible for every clinical decision.

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Frequently asked questions