Pyrethrin and pyrethroid flea products are the most common poisoning call in cats that live with dogs, and the mistake is almost always the same: a dog spot-on applied to a cat, or a cat grooming a treated dog. Signs start within 2 hours, mostly tremors and drooling, and a bath in the first 2 hours removes most of the dose still sitting on the skin. Cats handle these compounds badly for a metabolic reason, not because they are small.
Why a cat is not a small dog
Cats have limited glucuronidation capacity. The liver clears pyrethroids by oxidation followed by conjugation with glucuronic acid, and the feline enzyme system for that second step is far weaker than the dog system. The compound therefore stays in the cat longer, and the tremor phase lasts 24 to 72 hours instead of a few hours. This is the same reason cats are fragile with acetaminophen and several NSAIDs.
Weight makes it worse. A 45 to 65 percent permethrin spot-on is dosed for a 20 to 40 kg dog. One tube on a 4 kg cat delivers roughly 10 times the mg per kg a large dog receives, and the cat clears it 5 to 10 times more slowly. The arithmetic alone explains why 1 tube can kill.
Human products count too. Lice shampoo for people contains 1 percent permethrin, scabies cream 5 percent, and even the 0.33 percent pyrethrin wash is not for cats. Ant powders, mosquito coils, and garden sprays labeled as natural pyrethrum are the same family.
The 2-hour line
What you do in the first 2 hours changes the course more than anything the clinic can do later. Skin absorption continues for hours while the product sits in the fur and sebum, so the goal is removal, not dilution. Bathe with a liquid dish soap, not a pet shampoo, from the chin down so water does not run into the eyes and mouth, lather for 3 to 5 minutes, rinse, and repeat once. Two washes remove more than 1. Dry with a towel and keep the cat warm; wet fur plus tremors plus a cool room drops body temperature fast.
Do not use flea shampoo, do not use a medicated shampoo, and do not wait to see whether signs appear before washing. If the cat is already trembling, still wash, then drive. If the product went into the mouth, rinse the mouth with 10 to 20 mL of water by syringe into the cheek pouch, not down the throat.
| Product applied | Typical strength | Risk to a 4 kg cat | First action |
|---|---|---|---|
| Dog spot-on, permethrin | 45 to 65 percent | high, tremors in 2 to 6 hours | bath within 2 hours, then vet |
| Dog flea shampoo | 0.2 to 0.5 percent | moderate, drooling in 2 to 4 hours | bath within 2 hours, then call |
| Cat spot-on, etofenprox | 40 percent, cat label | low, monitor 2 to 4 hours | no bath, observe 2 hours |
| Human lice shampoo | 1 percent permethrin | moderate, signs in 4 to 8 hours | bath within 2 hours, then call |
| Household spray, cypermethrin | 0.1 to 0.5 percent | low after 4 hours drying | ventilate, cat out 4 hours |
What poisoning looks like
The classic picture is a cat with twitching ears, flicking facial muscles, drooling more than usual, and a stiff, unsteady walk. Progression runs from fine muscle fasciculation to whole-body tremor, then to hyperthermia above 40 degrees Celsius, then to seizures in the worst cases. The tremors generate heat, the cat cannot pant effectively, and a 40.5 degree reading 6 hours in is a genuine emergency on its own. Local skin redness and itching at the application site are common and less serious than the neurological signs.
Low-grade cases look like a cat that is restless, keeps shaking its head, and hides. Owners often read that as a skin reaction. If the cat had any pyrethroid contact in the past 24 hours, treat restlessness as a neurological sign.
What the clinic does
Further decontamination if the bath at home was incomplete, intravenous fluids to support circulation and protect the kidneys, and active cooling with wet towels and fans when temperature passes 40 degrees Celsius, never with ice. Tremor control is methocarbamol 凭兽医处方, sometimes diazepam, and seizure control with a benzodiazepine if needed. Bloodwork checks muscle breakdown, kidney values, and glucose. There is no antidote, and none is needed in most cases: with early decontamination, the majority of cats are eating and walking normally again within 24 to 72 hours.
Prognosis turns on 2 numbers: how many hours passed before the bath, and the peak temperature. Under 2 hours and under 40 degrees, the outlook is good. Over 6 hours with a temperature above 41 degrees and seizure activity, survival drops sharply.
Why the label is not a suggestion
Never split a dog tube for a cat, never halve the dose by eye, and never use a dog product on a cat because the cat is large. The cat-labeled version of the same ingredient exists at 0.5 to 3 percent, which is 20 to 100 times lower than the dog spot-on. If a dog in the house is treated, keep the cat away for 24 hours, until the application site is dry, and do not let the cat sleep on the dog's bedding that night.
Read the ingredient list before purchase: permethrin, cypermethrin, deltamethrin, fenvalerate, and etofenprox are all pyrethroids, and only the last one is routinely sold in a cat-safe formulation. Products that say for dogs over 20 kg or for dogs only are a hard stop.
Household rules
- One cupboard for cat products, another for dog products, and never the same shelf.
- Write the dog's treatment date on the box; the 24 hours after application is the risk period.
- Never treat a cat with a product whose label lacks a cat dose in mL per kg.
- Keep the shampoo bottle used for decontamination in the bathroom, not the garden shed.
- If 2 cats live together, treat neither with a dog product and wash both if one is contaminated through grooming.
When to drive now
Drive now for any tremor, for drooling that started within 12 hours of a flea product, for a temperature at or above 40 degrees Celsius, for a cat that cannot walk straight, or for any seizure. Also drive if a kitten under 6 months was exposed, because the same mg per kg hits harder and the margin is smaller. Call on the way and say the word permethrin: practices triage it faster than a description of shaking.
The same short-window logic applies to zinc coin ingestion and vitamin D exposure; the toxicity lookup tool lists ingredient thresholds by species.
Frequently asked
I used a dog flea spot-on on my cat by mistake. How long do I have?
Bathe within 2 hours with liquid dish soap, 2 washes, chin down, then call the vet whether or not signs have started. Signs usually begin 2 to 6 hours after application and can run 24 to 72 hours because cats clear pyrethroids slowly. Do not wait for the first tremor before washing.
Is a pyrethrin shampoo as risky as a permethrin spot-on?
Less risky, but not safe. Dog shampoos at 0.2 to 0.5 percent usually cause drooling and unsteadiness rather than seizures, while a 45 to 65 percent spot-on can be lethal to a 4 kg cat. Both need a bath and a call, and human lice shampoo at 1 percent is also off limits for cats.
Can I use half a dog tube to save money?
No. Dosing by eye on a 20 to 40 kg tube for a 4 kg cat is how most of these cases start, and the concentration is 20 to 100 times the cat-labeled product. Buy a cat-labeled product with a dose in mL per kg, and keep dog and cat supplies on separate shelves.