Fomepizole (4-methylpyrazole)

What is this medication for?

• Known/suspected ethylene glycol (antifreeze) toxicity
• Ethylene glycol toxicity — high-dose regimen, only if within 3 hours of ingestion
• Methanol toxicity (human label; same ADH logic)

How is it given?

This medication is normally given by your veterinary team (in clinic or hospital).

What if I miss a dose?

• Give it as soon as you remember — unless it is almost time for the next dose (then skip the missed one; never double up).
• Call your clinic if you miss more than one dose in a row.

Side effects — what should I watch for?

Effects listed as common in this handout

• Vein irritation/phlebosclerosis (rapid concentrated into a vein) (Administration-related)
• Mild drowsiness (Feline high-dose)

Worth a phone call to your veterinarian

• Any new symptom that worries you.

⚠️ Contact your veterinarian or an emergency clinic IMMEDIATELY if you notice

• Anaphylaxis (1 dog in trials) (Rare)

Before starting (or refilling), tell your veterinarian if your pet:

• has or has had: Not a monotherapy — correct acid-base/electrolytes, fluids ± dialysis; emesis/lavage + charcoal if within 1–2 h; azotemic dogs = poor prognosis (still treat up to 36 h)

• is pregnant, nursing, or used for breeding
• is taking any other medications, supplements, or flea/tick products, including:
  • Ethanol

Storage and handling

Dilute per kit directions; rapid undiluted into a vein → vein irritation/phlebosclerosis. Falsifies some EG screening tests (as does ethanol).

Good to know

• The cat-dog dose asymmetry is the exam gold: dogs 20 mg/kg load, cats 125 mg/kg load (cats need ~6× more) — and cats must be treated within 3 h (4 h = 100% mortality in the Connally study); cats treated late had zero survivors regardless of therapy.
• Ethylene glycol itself is only mildly toxic — the METABOLITES kill (metabolic acidosis + oxalate crystal tubular tissue death). Block ADH and the parent compound flows out unchanged in urine.
• Fomepizole beats ethanol as antidote: no CNS depression/hypoglycemia, easier monitoring — ethanol (7% solution CRI) remains the fallback when fomepizole is unavailable/too expensive.
• Both antidotes can false-positive EG test kits — confirm with blood EG level or osmolar gap when possible.