Metronidazole

Metronidazole is a nitroimidazole antibacterial and antiprotozoal agent used for susceptible anaerobic infections and selected protozoal diseases. Veterinary applications include enteric and extraintestinal infections in companion animals and anaerobic infections in horses. Its established antimicrobial targets provide a different clinical rationale from the less consistently supported empirical treatment of nonspecific diarrhea.

Group

Nitroimidazole antibacterial and antiprotozoal

Mechanism of Action

Metronidazole enters susceptible cells and undergoes reductive activation in low-redox intracellular environments. Reactive intermediates damage DNA and interfere with nucleic-acid function, inhibiting replication and leading to microbial death. Dependence on reductive activation explains its principal activity against anaerobic bacteria and selected protozoa. Metronidazole benzoate is converted to active metronidazole after administration.

Dose Rates

Dogs

Use / stage	Dose	Route	Schedule	Instructions
Anaerobic infection	15 mg/kg	PO	Every 12 h
Anaerobic infection	12 mg/kg	PO	Every 8 h
Anaerobic infection	15 mg/kg	IV		Diluted slow infusion over at least 30 minutes; the IV reported instruction does not state a repeat interval.
Giardiasis	12–15 mg/kg	PO	Every 12 h	Up to 7 days; Course 5-7 days.
Giardiasis	15–25 mg/kg	PO	Every 12–24 h	Up to 7 days; Course 5-7 days; do not combine with other metronidazole regimens.
Giardiasis	22 mg/kg	PO	Every 12 h	Up to 5 days; May be combined with fenbendazole 50 mg/kg daily for 3 or 5 days when indicated.
Entamoeba or Pentatrichomonas infection	25 mg/kg	PO	Every 12 h	Up to 8 days
Anaerobic sepsis	15 mg/kg	IV	Every 12 h	Slow properly prepared IV infusion.
Clostridium perfringens enterotoxicosis	10–20 mg/kg	PO	Every 12 h	Up to 28 days; Course 7-28 days; confirm a clinically relevant diagnosis.
Large-bowel inflammatory disease adjunct	10–20 mg/kg	PO	Every 8–12 h	Up to 28 days; Historical adjunct for selected refractory cases; maximum listed exposure is 60 mg/kg/day. Diet and diagnostic work-up remain important.
Inflammatory bowel disease adjunct	10–15 mg/kg	PO	Every 8–12 h	Combined with appropriate anti-inflammatory management in moderate/severe disease.
Refractory lymphoplasmacytic enteritis	Reference only	PO		10-30 mg/kg every 8-24 hours for 2-4 weeks is a broad historical recommendation; the highest amount at the shortest interval exceeds the stated mammalian daily ceiling, so amounts and intervals must not be selected independently.
Historical tear-staining treatment	Reference only	PO		100-200 mg/animal daily for 10 days each month; this intermittent historical use does not establish an appropriate universal dose or routine need for antibiotics.

Cats

Use / stage	Dose	Route	Schedule	Instructions
Anaerobic infection	10–25 mg/kg	PO	Every 24 h
Giardiasis	17 mg/kg	PO	Every 24 h	Up to 8 days; Use the weight-based amount; the “one-third tablet” example lacks a specified strength and weight.
Giardiasis with benzoate	Reference only	PO		Metronidazole benzoate 25 mg/kg every 12 hours for 7 days; different ingredient basis from metronidazole. Do not calculate this amount through a base-strength tablet.
Giardiasis	15–25 mg/kg	PO	Every 12–24 h	Up to 7 days; Course 5-7 days; do not combine with other metronidazole regimens.
Giardiasis	22 mg/kg	PO	Every 12 h	Up to 5 days; May be combined with fenbendazole 50 mg/kg daily for 3 or 5 days when indicated.
Giardiasis	25 mg/kg	PO	Every 12 h	Up to 7 days
Entamoeba or Pentatrichomonas infection	25 mg/kg	PO	Every 12 h	Up to 8 days
Anaerobic sepsis	15 mg/kg	IV	Every 12 h	Slow properly prepared IV infusion.
Large-bowel inflammatory disease adjunct	10–20 mg/kg	PO	Every 8–12 h	Up to 28 days; Historical adjunct for selected refractory cases; maximum listed exposure is 60 mg/kg/day. Diet and diagnostic work-up remain important.
Inflammatory bowel disease adjunct	10–15 mg/kg	PO	Every 8–12 h	Combined with appropriate anti-inflammatory management in moderate/severe disease.
Diet-associated IBD adjunct	62.5 mg/animal	PO	Every 24 h	Up to 20 days; Course 10-20 days with dietary treatment; monitor dose relative to body weight.
Hepatic encephalopathy adjunct	7.5 mg/kg	PO	Every 8–12 h

Horses

Use / stage	Dose	Route	Schedule	Instructions
Anaerobic or protozoal infection	10 mg/kg	PO	Every 12 h
Higher-dose anaerobic infection treatment	Reference only	PO		15-20 mg/kg every 6 hours has been used, but totals 60-80 mg/kg/day and may exceed the monograph’s own 50-65 mg/kg/day ceiling. Retained as a conflicting high-risk reported alternative, not an established safe treatment recommendation.
Foal anaerobic infection	10–15 mg/kg	IV, PO	Every 12 h	Foals have slower elimination than adult horses.
Clostridial colitis	15 mg/kg	PO	Every 8 h
Bacteroides-associated metritis	15–25 mg/kg	PO	Every 12 h
Foal clostridial diarrhoea	25 mg/kg	PO	Every 12 h
Foal Clostridium perfringens infection	10–15 mg/kg	PO	Every 6–8 h	Foals only; interval depends on severity.
Foal Clostridium perfringens infection	10 mg/kg	IV	Every 6 h	Foals with ileus and oral intolerance; properly prepared slow infusion.
Foal anaerobic IV course — Loading	15 mg/kg	IV
Foal anaerobic IV course — Maintenance	7.5 mg/kg	IV	Every 6 h
Higher-dose anaerobic treatment alternative	Reference only	PO, RECTAL		20-25 mg/kg every 8-12 hours is listed, but the highest/shortest combination gives 75 mg/kg/day, exceeding the same guidance’s 65 mg/kg/day ceiling. Rectal absorption is lower; do not assume equivalent exposure.

Cattle

Use / stage	Dose	Route	Schedule	Instructions
Historical bull trichomoniasis treatment	Reference only	IV		75 mg/kg every 12 hours for three doses is described but greatly exceeds the same monograph’s daily ceiling. It is not a recommended treatment, and treated cattle must not enter the food chain under the applicable restrictions.

Ferrets

Use / stage	Dose	Route	Schedule	Instructions
Susceptible infection	10–30 mg/kg	PO	Every 12–24 h	Bitter taste; mask palatably without changing amount.
Inflammatory bowel disease adjunct	50 mg/kg	PO	Every 24 h

Rabbits

Use / stage	Dose	Route	Schedule	Instructions
Anaerobic infection	20 mg/kg	PO	Every 12 h	Up to 5 days; Course 3-5 days.
Anaerobic infection	40 mg/kg	PO	Every 24 h
Anaerobic infection	5 mg/kg	IV	Every 12 h	Slow IV.

Chinchillas

Use / stage	Dose	Route	Schedule	Instructions
Susceptible infection	10–40 mg/kg	PO	Every 24 h
High-dose giardiasis treatment	Reference only	PO		50-60 mg/kg twice daily for 5 days is reported, but totals 100-120 mg/kg/day and conflicts with the other source’s all-species daily ceiling. A specific exotic-animal assessment is required.
Broad historical infection regimen	Reference only	PO		20-60 mg/kg every 8-12 hours is reported; many dose/interval combinations exceed the cited daily ceiling and must not calculate as a freely selectable range. Mouse drinking-water and fixed rodent examples depend on intake or body-size qualification.

Guinea pigs

Use / stage	Dose	Route	Schedule	Instructions
Susceptible infection	10–40 mg/kg	PO	Every 24 h
Broad historical infection regimen	Reference only	PO		20-60 mg/kg every 8-12 hours is reported; many dose/interval combinations exceed the cited daily ceiling and must not calculate as a freely selectable range. Mouse drinking-water and fixed rodent examples depend on intake or body-size qualification.

Mice

Use / stage	Dose	Route	Schedule	Instructions
Broad historical infection regimen	Reference only	PO		20-60 mg/kg every 8-12 hours is reported; many dose/interval combinations exceed the cited daily ceiling and must not calculate as a freely selectable range. Mouse drinking-water and fixed rodent examples depend on intake or body-size qualification.

Rats

Use / stage	Dose	Route	Schedule	Instructions
Broad historical infection regimen	Reference only	PO		20-60 mg/kg every 8-12 hours is reported; many dose/interval combinations exceed the cited daily ceiling and must not calculate as a freely selectable range. Mouse drinking-water and fixed rodent examples depend on intake or body-size qualification.

Gerbils

Use / stage	Dose	Route	Schedule	Instructions
Broad historical infection regimen	Reference only	PO		20-60 mg/kg every 8-12 hours is reported; many dose/interval combinations exceed the cited daily ceiling and must not calculate as a freely selectable range. Mouse drinking-water and fixed rodent examples depend on intake or body-size qualification.

Hamsters

Use / stage	Dose	Route	Schedule	Instructions
Broad historical infection regimen	Reference only	PO		20-60 mg/kg every 8-12 hours is reported; many dose/interval combinations exceed the cited daily ceiling and must not calculate as a freely selectable range. Mouse drinking-water and fixed rodent examples depend on intake or body-size qualification.

Birds

Use / stage	Dose	Route	Schedule	Instructions
Anaerobic infection or giardiasis	Reference only	PO		10-50 mg/kg every 12 hours is reported; exposure can reach 100 mg/kg/day, conflicting with the earlier all-species ceiling. Ratite alternative 20-25 mg/kg twice daily is described only for non-food birds.

Reptiles

Use / stage	Dose	Route	Schedule	Instructions
Anaerobic or protozoal treatment	Reference only	PO		Reported alternatives include 100 mg/kg repeated after 2 weeks, or 50 mg/kg daily for 3-5 days; reptile amoeba/flagellate regimens include 100-275 mg/kg repeated in 1-2 weeks, and 150 mg/kg weekly for anaerobes. Drymarchon and selected Lampropeltis have a lower 40 mg/kg two-week-repeat protocol. Species tolerance and conflicts with the earlier all-species ceiling prevent a universal calculator regimen.

Amphibians

Use / stage	Dose	Route	Schedule	Instructions
Anaerobic or protozoal treatment	Reference only	PO		Reported alternatives include 100 mg/kg repeated after 2 weeks, or 50 mg/kg daily for 3-5 days; reptile amoeba/flagellate regimens include 100-275 mg/kg repeated in 1-2 weeks, and 150 mg/kg weekly for anaerobes. Drymarchon and selected Lampropeltis have a lower 40 mg/kg two-week-repeat protocol. Species tolerance and conflicts with the earlier all-species ceiling prevent a universal calculator regimen.

Indications

Treatment of susceptible anaerobic bacterial infections in dogs, cats and other veterinary patients, including selected oral, abdominal, soft-tissue or systemic infections.
Treatment of selected anaerobic infections in horses, including susceptible Clostridium or Bacteroides infections.
Treatment of Giardia infection in selected dogs or cats with a supported regimen, with variable parasite clearance.
Combination treatment of selected Helicobacter-associated gastric disease when an organism-directed regimen is justified.

Contraindications

Use cautiously during pregnancy and with impaired hepatic clearance.
Food-producing animals: reported guidance prohibits nitroimidazole administration.

Interactions

Can increase warfarin and cyclosporine effects through metabolic interactions.

Adverse effects

Bitter taste, vomiting and anorexia
Dose-related ataxia, depression, tremor, seizures and peripheral neuropathy
Possible hepatic injury and impaired canine scent detection

Toxicity

Stop treatment for neurotoxicity. Benzodiazepine-responsive signs are described; reported guidance treatment uses diazepam 0.4 mg/kg every 8 hours for 3 days, with supportive care.

Preparations

Preparation	Form	Strength	Route
Metronidazole 250 mg tablet	Tablet	250 mg / tablet	PO
Metronidazole 500 mg tablet	Tablet	500 mg / tablet	PO
Metronidazole 375 mg capsule	Capsule	375 mg / capsule	PO
Metronidazole 50 mg/mL oral suspension	Oral Liquid	50 mg / mL	PO
Metronidazole 5 mg/mL injection Ready-to-infuse products and acidic concentrates require their own instructions; do not assume identical preparation steps.	Injection	5 mg / mL	IV
Metronidazole benzoate 16 mg/mL compounded suspension Concentration of benzoate ester, not metronidazole base; approximately 62% base by mass.	Oral Liquid	16 mg / mL	PO

Monitoring and Administration

Monitor neurologic signs and cumulative daily exposure. The reported daily ceiling is inconsistently stated as 50-65 mg/kg/day; avoid treating it as one exact universal safe maximum.

Metronidazole benzoate is approximately 62% metronidazole by mass. A 25 mg/kg benzoate dose supplies approximately 15.5 mg/kg metronidazole; do not equate the two ingredient identities.

The instructions for acidic metronidazole hydrochloride concentrate must not be applied automatically to a commercial ready-to-infuse 5 mg/mL bag. For the specifically described acidic concentrate, dilution and pH adjustment require product-specific pharmacy instructions.

Do not extrapolate particular compounded stability to all vehicles. Particular Ora-Plus/Ora-Sweet formulations were stable for 90 days; other mixtures can lose potency within 28 days.

Helicobacter combinations: dogs metronidazole 15.4 mg/kg every 8 hours, amoxicillin 11 mg/kg every 8 hours and bismuth subsalicylate 0.22 mL/kg every 4-6 hours for 3 weeks; cats metronidazole 10-15, clarithromycin 7.5 and amoxicillin 20 mg/kg each twice daily for 14 days. Ferret alternatives use metronidazole/amoxicillin 22 mg/kg plus bismuth 17.6 mg/kg three times daily for 3-4 weeks, or metronidazole 20, amoxicillin 30 and bismuth 7.5 mg/kg every 8 hours for 21-28 days. Product composition and organism identification matter.

Foal 15-25 mg/kg every 8 hours can exceed the daily ceiling at its upper end; the separate 25 mg/kg twice-daily option does not. Lawsonia combination therapy describes metronidazole 10-15 mg/kg every 8-12 hours with oxytetracycline 10-18 mg/kg slow IV daily or chloramphenicol 44 mg/kg PO every 6-8 hours. In major hepatic impairment, a 25-50% usual-dose strategy is described; adjust clinically rather than apply an automatic percentage to every indication. Avoid aluminium-hub needles with acidic concentrate.