Fomepizole (4-MP): veterinary dose and monograph

At a glance

Drug class

Veterinary drug

Also known as

4-methylpyrazole, Antizol, Antizol-Vet

Species with doses

Dogs and Cats

General dose range

20 mg/kg IV tapering schedule (0, 12, 24, 36 h) — check the species tables for the indication

Routes

IV

Last reviewed

1 October 2026

Indications

• Ethylene glycol (antifreeze) toxicosis — dogs and cats (the earlier, the better)
• Suspected antifreeze ingestion pending confirmation (treat on suspicion — minutes matter)

Fomepizole (4-MP) 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

• Ethylene glycol toxicosis: Papich 5e and Plumb’s p.554 (Antizol-Vet label): 20 mg/kg IV, then 15 mg/kg at 12 and 24 h, then 5 mg/kg at 36 h; extra 5 mg/kg doses if ethylene glycol is still present. Most effective within 8 h of ingestion.

Cats

• Ethylene glycol toxicosis (HIGH-dose feline protocol): Papich 5e and Plumb’s p.554 (Connally): 125 mg/kg slow IV, then 31.25 mg/kg at 12, 24 and 36 h; continue q12h until ethylene glycol is undetectable (or through 60 h). Cats must be treated within 3 h of ingestion — at 4 h mortality was 100%.

Contraindications

Known hypersensitivity. Late presentation with established anuric renal failure (antidote alone won't reverse it — needs dialysis/intensive care). Not effective once ethylene glycol is fully metabolised. Use the correct (much higher) feline protocol — standard canine dosing fails in cats.

Species-specific warnings

Cats: Use the high-dose feline protocol (loading ~125 mg/kg) and treat as early as possible (ideally <3 h post-ingestion) — cats poisoned by ethylene glycol have a grave prognosis if treatment is delayed.

Adverse effects

• Generally well tolerated (a major advantage over ethanol)
• Occasional anaphylactoid reaction / hypersalivation with rapid infusion
• Mild CNS effects
• (Cats: high-dose protocol — monitor)

Drug interactions

• Ethanol: the alternative ADH inhibitor — do not use both (additive CNS depression); fomepizole is preferred
• Drugs metabolised by alcohol dehydrogenase: handling altered
• No major other interactions in the acute setting

Pregnancy and lactation

In life-threatening antifreeze poisoning the antidote is given regardless; specific reproductive data limited.

Monitoring

• Renal values (BUN/creatinine), urine output and urinalysis (calcium oxalate crystals)
• Acid–base status (metabolic acidosis) and electrolytes
• Ethylene glycol level / clinical course where available
• Time since ingestion (the dominant prognostic factor)

Toxicity and overdose

Very well tolerated — its safety (no sedation/hyperosmolar effects) is a key advantage over ethanol therapy. The "failure," not toxicity, is the main risk: given too late (after metabolism/renal injury), or at standard canine doses in a cat, it does not save the patient.

Management: Rarely an issue given its wide margin; supportive care if needed. The real management priority is the poisoning itself — combine the antidote with IV fluids, correction of acidosis, and renal monitoring; consider haemodialysis for late/anuric cases.

Clinical pearls

• Antifreeze poisoning is a race against metabolism — give fomepizole on SUSPICION; every hour of delay worsens the prognosis
• It blocks the enzyme that turns ethylene glycol into kidney-destroying acids — so it works only before that conversion/renal injury occurs
• CATS need a MUCH higher dose and even earlier treatment than dogs — standard canine dosing fails in cats
• Preferred over ethanol (no sedation, no hyperosmolality, easier monitoring)
• Warm the product if it has crystallised, and pair the antidote with fluids/acidosis correction

Mechanism of action

A competitive inhibitor of alcohol dehydrogenase, the enzyme that initiates ethylene glycol metabolism. By blocking this first step it prevents formation of the toxic downstream metabolites (glycolaldehyde, glycolic and oxalic acid) responsible for the severe metabolic acidosis and calcium-oxalate renal tubular injury — allowing the parent ethylene glycol to be excreted largely unchanged. It works only while unmetabolised ethylene glycol remains.

Formulations and products

• Antizol-Vet 1.5 g kit added to the 30 mL 0.9% NaCl supplied = 50 mg/mL (Plumb’s p.554; Papich 5e p.392)
• Injection 1 g/mL concentrate (human, dilute before use)
• Antizol-Vet (Various) — 1.5 g kit — add to the 30 mL 0.9% NaCl supplied = 50 mg/mL (Plumb’s p.554), Injection
• Antizol / Fomepizole — 1 g/mL concentrate (dilute before use), Injection concentrate (human-label)

Storage

Antizol-Vet: store per label (may need warming if it solidifies — fomepizole can crystallise when cool; warm to redissolve before use). Dilute concentrate before IV administration.

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