Antifreeze (Ethylene Glycol)

How Much Is Dangerous

Extremely small — as little as one to two teaspoons (roughly 4-8 mL) can be lethal to an average-sized cat. This is one of the most acutely dangerous common household substances for cats gram for gram, on par with or exceeding some of the most dangerous items on this entire list, and the small lethal volume means even a minor garage floor spill or a small leak from a parked car represents a genuine, serious risk.

Symptoms

• Stage 1 (30 minutes to 12 hours after ingestion): appears drunk or uncoordinated, vomiting, excessive thirst and urination, and in some cases seizures — this stage reflects the direct CNS-depressant effect of the unmetabolized ethylene glycol itself, similar in character to alcohol intoxication
• Stage 2 (12–24 hours after ingestion): apparent improvement as the acute neurological signs subside — this is genuinely deceptive, not recovery, and represents the cat moving from the "drunk" phase into the phase where toxic metabolites are actively accumulating and beginning to damage the kidneys
• Stage 3 (24–72 hours after ingestion in cats, often faster than the timeline typically seen in dogs): kidney failure sets in in earnest — lethargy, loss of appetite, vomiting, reduced or completely absent urine output, seizures, and coma as the calcium oxalate crystals accumulate and destroy kidney tissue
• Cats can progress to fatal kidney failure faster than dogs typically do with an equivalent exposure, sometimes within 24 hours of ingestion, which is part of why this specific poisoning is treated with even greater urgency in cats than the already-urgent standard for dogs

Time to Symptoms

Initial neurological signs within 30 minutes to a few hours of ingestion; the deceptive apparent-recovery period around 12–24 hours; established, often fatal kidney failure develops within 24–72 hours of ingestion, frequently on the faster end of that range in cats specifically.

First Aid Steps

• This is one of the most extreme, unambiguous veterinary emergencies on this entire list — go to an emergency animal hospital immediately, including overnight if needed, without waiting for a regular clinic to open
• Do not wait for symptoms to develop before seeking care — the treatment window during which the antidote is genuinely effective is measured in hours, and by the time kidney failure becomes visible, the antidote is often no longer able to change the outcome
• If you witnessed the exposure directly, or found a spill/leak with the cat nearby (even without directly seeing them drink it), say so explicitly and immediately to the veterinary team — with this specific poison, early treatment before any kidney damage has begun is what actually saves lives, and every hour matters more here than with almost any other toxin on this list

What Not To Do

• Do not wait to "see if the cat seems okay" during or after the apparent-improvement stage 2 window — this is not a sign of safety, it is a well-documented, dangerous feature of this specific poisoning's timeline
• Do not assume a small spill on a garage floor is a minor cleanup issue rather than a genuine pet-safety emergency — the lethal amount for a cat is genuinely tiny, smaller than most people intuitively expect
• Never use human antifreeze antidotes, home remedies, or attempt to induce vomiting as a substitute for veterinary care — this requires an IV antidote given under medical supervision, with no meaningful safe home-based intervention available
• Do not delay seeking care to "wait and see" even briefly — of everything on this list, this is the poison where the phrase "every minute counts" is least exaggerated

Veterinary Treatment

Time-critical antidote therapy: fomepizole (4-methylpyrazole), or ethanol where fomepizole is unavailable, works by blocking alcohol dehydrogenase — the same enzyme responsible for converting ethylene glycol into its toxic metabolites — but is only effective if given before significant metabolism has already occurred, ideally within a few hours of ingestion and essentially never useful once established kidney damage has set in. Once kidney damage has occurred, treatment shifts to supportive care (aggressive IV fluids) with a substantially poorer prognosis; peritoneal dialysis or hemodialysis, where available at specialty referral centers, may be considered in advanced cases but carries a guarded outlook even then. Prognosis is good with antidote treatment started within about 8 hours of ingestion and becomes progressively worse, then genuinely poor, after that window closes.

Toxic Compound

Ethylene glycol itself is only mildly toxic — what actually destroys the kidneys is a cascade of toxic metabolites the liver produces while breaking it down. The enzyme alcohol dehydrogenase converts ethylene glycol first to glycoaldehyde, then to glycolic acid, then to glyoxylic acid, and finally to oxalic acid, which combines with calcium in the blood to form calcium oxalate crystals that lodge in and physically destroy the kidney's filtering tubules. This same metabolic pathway is the target of the antidote (see Treatment) — blocking the first enzymatic step prevents the whole toxic cascade from happening. Its sweet taste, from the ethylene glycol itself, is part of why it is so commonly involved in accidental poisonings of pets, who will readily drink spilled or leaked antifreeze that a human would recognize and avoid by smell.