Metronidazole
2-(2-methyl-5-nitro-1H-imidazol-1-yl)ethanol; Metronidazole benzoate
Also known as: Flagyl · Metrobactin · Stomorgyl · Metronidazole benzoate · Bayer-5360 · metronidazolum · SC-32642 · NSC-50364 · RP-8823 · SC-10295
Reviewed by the VetSheet Veterinary TeamUpdated Apr 5, 2026Evidence-based veterinary reference
Audited v13 dosing evidence
Structured calculator evidenceAnaerobic infections (extra-label) — less severe anaerobic infections
- q8-12h
NA
Governing clinical context (6)
- 1. Dosages are for metronidazole base unless otherwise noted. If metronidazole benzoate is being used, dosages should be adjusted unless provided by pharmacy as “mg/mL of the base.”
- One mg of metronidazole base = 1.6 mg of metronidazole benzoate. For example, if the calculated metronidazole dose is 200 mg (of the base), the corresponding dose using metronidazole benzoate would be 320 mg (metronidazole benzoate).
- 2. Because of the drug’s extreme bitterness, using a flavored, compounded metronidazole benzoate or putting quartered tablets in an empty gelatin capsule may be considered.
- Anaerobic infections (extra-label):
- ■ Give this medication with food.
- Metronidazole tablets and solution for injection should be stored at temperatures less than 30°C (86°F) and protected from light. The shelf life of divided tablets is 3 days. 5
Hepatic encephalopathy (extra-label)
- q8-12h
NA
Governing clinical context (5)
- 1. Dosages are for metronidazole base unless otherwise noted. If metronidazole benzoate is being used, dosages should be adjusted unless provided by pharmacy as “mg/mL of the base.”
- One mg of metronidazole base = 1.6 mg of metronidazole benzoate. For example, if the calculated metronidazole dose is 200 mg (of the base), the corresponding dose using metronidazole benzoate would be 320 mg (metronidazole benzoate).
- 2. Because of the drug’s extreme bitterness, using a flavored, compounded metronidazole benzoate or putting quartered tablets in an empty gelatin capsule may be considered.
- ■ Give this medication with food.
- Metronidazole tablets and solution for injection should be stored at temperatures less than 30°C (86°F) and protected from light. The shelf life of divided tablets is 3 days. 5
Doses are a clinical reference for licensed veterinary professionals. Always confirm against the current label and the individual patient.
Overview
Metronidazole is a synthetic nitroimidazole antibacterial and antiprotozoal agent widely used in veterinary medicine.
Key Clinical Applications:
- Antiprotozoal: Highly effective against Giardia, Trichomonas, Entamoeba histolytica, and Balantidium coli.
- Anaerobic Antibacterial: Excellent efficacy against obligate anaerobes (e.g., Bacteroides, Clostridium, Fusobacterium). It is frequently used for enteric and systemic anaerobic infections, including sepsis, peritonitis, and deep bite wounds.
- Gastrointestinal Modulator: Frequently utilized in the management of Inflammatory Bowel Disease (IBD) and chronic colitis. Clinical Pearl: Beyond its antimicrobial properties, metronidazole is believed to have local immunomodulatory and anti-inflammatory effects in the GI tract, specifically by inhibiting cell-mediated immunity.
Important Considerations:
- Strictly prohibited by the FDA for use in food-producing animals.
- Notorious for its extremely bitter, metallic taste, which can cause profound hypersalivation and dosing aversion, especially in cats. Compounded metronidazole benzoate is often preferred for feline patients as it is less soluble in saliva and therefore less bitter.
Mechanism of action
Metronidazole is a concentration-dependent bactericidal agent.
Mechanism Pathway:
- Entry: The drug enters the cell of the organism via passive diffusion.
- Activation: In obligate anaerobes and certain protozoa, the nitro group of metronidazole is reduced by electron-transport proteins (such as ferredoxin).
- DNA Disruption: This reduction process creates highly reactive, short-lived cytotoxic intermediate compounds (free radicals) → These radicals bind to and disrupt DNA and nucleic acid synthesis → leading to rapid cell death.
Because aerobic bacteria lack the specific electron-transport proteins required to reduce and activate the drug, metronidazole is virtually inactive against obligate aerobes.
Safety & warnings
Contraindications
- Hypersensitivity to metronidazole or other nitroimidazole derivatives
- Food-producing animals (FDA prohibited)
- Pregnancy (potential teratogen, especially in early pregnancy)
- Severe hepatic disease (requires dose reduction)
Adverse effects
- Neurologic disorders (ataxia, nystagmus, seizures, head-tilt)
- Lethargy and weakness
- Neutropenia
- Hepatotoxicity
- Hematuria
- Anorexia, nausea, vomiting, and diarrhea
- Hypersalivation (especially in cats due to bitter taste)
- CNS toxicity (nystagmus, ataxia, knuckling, head tilt, seizures)
- Haematuria
- Excessive salivation/foaming (especially in cats)
- Impaired sense of smell (reported in explosive detection dogs)
Precautions
Neurotoxicity Warning: In dogs, total daily doses should not exceed 65 mg/kg/day to minimize the risk of central vestibular and cerebellar toxicity. Toxicity can occur with acute high doses or chronic moderate-to-high dose therapy.
- Hepatic Impairment: Use with caution in animals with hepatic dysfunction. Consider reducing the dose to 25-50% of the usual dose in significant liver impairment.
- Pregnancy: Potential teratogen. Avoid use during pregnancy, particularly during the first 3 weeks of gestation, unless benefits clearly outweigh risks.
- Nursing: Consider alternative therapy or switching to milk replacer due to potential tumorigenicity.
- Debilitated Patients: Use with extreme caution in severely debilitated animals.
Drug interactions
May induce a disulfiram-like reaction (nausea, vomiting, cramps).
May decrease the metabolism of metronidazole, increasing the likelihood of dose-related side effects.
May increase the metabolism of metronidazole, thereby decreasing blood levels.
May increase the metabolism of metronidazole, thereby decreasing blood levels.
May prolong the prothrombin time (PT). Avoid concurrent use or intensify monitoring.
Should not be used concurrently with Spiramycin (in combination products) as the combination may be antagonistic.
Monitoring
- Clinical efficacy (resolution of diarrhea, negative fecal tests)
- Adverse effects (especially neurologic signs like ataxia or nystagmus)
- Neurological signs (for early detection of CNS toxicity)
- Liver function tests (ALT, AST, Bilirubin) in patients with pre-existing hepatic disease
- Complete Blood Count (for neutropenia)
Pharmacokinetics
Half-life
Absorption
Relatively well absorbed after oral administration. Oral bioavailability in dogs is high (50-100%) and enhanced with food. In cats, bioavailability averages ~65%. In horses, oral bioavailability averages ~80% (decreased by 50% if given rectally).
Distribution
Highly lipophilic and widely distributed to most body tissues and fluids, including bone, abscesses, the CNS, and seminal fluid. Less than 20% bound to plasma proteins.
Metabolism
Primarily metabolized in the liver via several pathways, including glucuronidation.
Elimination
Both metabolites and unchanged drug are eliminated in the urine and feces.
Overdose
Signs of intoxication in dogs and cats are primarily neurologic and include:
- Anorexia and/or vomiting
- Depression
- Mydriasis (dilated pupils)
- Nystagmus (rapid eye flicking, very common in dogs)
- Ataxia (stumbling, uncoordinated gait)
- Head-tilt, proprioceptive deficits, joint knuckling
- Tremors, seizures, rigidity, and stiffness
These effects may be seen with acute massive overdoses, or more commonly with chronic therapy at doses >60 mg/kg/day (though sometimes seen at 'recommended' doses like 30 mg/kg/day).
Treatment:
- Discontinue metronidazole immediately. Most dogs with mild to moderate signs improve rapidly within 1-2 days of stopping the drug.
- Diazepam has been used successfully to rapidly decrease the CNS effects associated with metronidazole toxicity.
- For acute massive ingestions, standard decontamination protocols (emesis, activated charcoal) apply, but use extreme caution before inducing vomiting if the patient is already showing CNS signs, due to aspiration risk.
Available products
Formulations
- Oral capsules
- Extended-release oral tablets
- Injection (IV)
- Compounded oral suspensions (often using metronidazole benzoate)
- 5 mg/ml i.v. infusion
- 250 mg tablet
- 500 mg tablet
- 25 mg metronidazole + 46.9 mg spiramycin tablet (Stomorgyl 2)
- 125 mg metronidazole + 234.4 mg spiramycin tablet (Stomorgyl 10)
- 250 mg metronidazole + 469 mg spiramycin tablet (Stomorgyl 20)
- 40 mg/ml oral solution
Veterinary
- None (Prohibited for use in food animals by the FDA)
- Metrobactin 250 mg, 500 mg tablets
- Stomorgyl 2, 10, 20 tablets (combination with spiramycin)
Human-labeled
- Metronidazole Oral Tablets: 250 mg & 500 mg (Flagyl, generic)
- Metronidazole Oral Capsules: 375 mg (Flagyl 375, generic)
- Metronidazole Extended-Release Oral Tablets: 750 mg (Flagyl ER)
- Metronidazole Injection: 5 mg/mL in 100 mL vials
- Bismuth Subsalicylate, Metronidazole & Tetracycline HCl Combination (Helidac)
- Flagyl 200 mg, 400 mg tablets
- Metronidazole 5 mg/ml IV infusion
Regulatory status
POM-V classification. Prohibited for use in food-producing animals.
POM-V classification.
No regulatory data for: 🇺🇸 US · 🇭🇰 HK · 🇹🇼 TW · 🇯🇵 JP · 🇰🇷 KR · 🇦🇺 AU
Storage & stability
Tablets and HCl powder for injection should be stored at temperatures less than 30°C and protected from light. The injection should be protected from light and freezing and stored at room temperature. Compounded metronidazole benzoate suspensions can be stable for up to 90 days or longer depending on the specific vehicle used.
Client information
Important Guidance for Pet Owners:
- Extremely Bitter Taste: Metronidazole tablets taste terrible to animals. Do not crush the tablets unless absolutely necessary, as the bitter taste will cause your pet (especially cats) to drool excessively or refuse food. If you are struggling to give the medication, ask your veterinarian about a compounded liquid version (metronidazole benzoate) which hides the taste much better.
- Give with Food: Giving this medication with a meal can help improve its absorption and reduce stomach upset.
- Watch for Neurologic Signs: Contact your veterinarian immediately if your pet shows any signs of neurologic toxicity. Look for: stumbling or "drunken" walking (ataxia), rapid back-and-forth eye movements (nystagmus), head tilt, extreme lethargy, tremors, or seizures.
- Complete the Course: Even if your pet's diarrhea or symptoms improve, finish the entire prescription to ensure the infection or parasite is fully cleared.
VetSheet drug reference is intended for licensed veterinary professionals as a clinical decision-support aid, not a substitute for professional judgement or the manufacturer’s current label.