Why a 3 kg cat can't handle even a drop
Cats lack the liver enzymes to break down permethrin, so what's harmless to a dog becomes a crisis for them. The spot-on formula is concentrated—a drop the size of a pinhead contains enough to overwhelm a small cat's system. Even if she licked it off the floor rather than directly from the dog, the risk is identical because absorption happens through mucous membranes.
Right now, her seeming normalcy is the most dangerous part. Permethrin's effects are delayed but sudden—like a switch flipping. The first neurological signs (twitching lips, ear flicking) often appear just as owners relax, thinking "it's been an hour and she's fine." By the time you see tremors, the toxin is already disrupting her nervous system.
What silent damage looks like in the first hour
Internally, permethrin overstimulates nerve cells, causing them to fire nonstop. At 30 minutes post-exposure, microscopic chaos is brewing even if her paws are still steady. Unlike toxins that cause vomiting or diarrhea as early warnings, permethrin gives no gastrointestinal hints—its first visible signs are neurological.
The concentration matters more than the volume. A "tiny drop" of 45% permethrin (common in dog spot-ons) delivers a massive dose relative to a cat's body weight. For perspective, the lethal dose for cats is roughly 1/10th of what's safe for a medium-sized dog—and spot-ons are formulated for dogs ten times her size.
Why you can't wait for symptoms
Permethrin toxicity moves faster than most owners expect. The progression from mild facial tremors to full-body seizures can take less than 90 minutes, and at that point, treatment becomes riskier and less effective. Waiting until she staggers or drools means losing critical time.
Emergency vets often administer intravenous lipids to soak up the toxin—a treatment that works best when given early. At home, there's no safe way to counteract the poisoning. Bathing her now won't help because the toxin is already absorbed, and inducing vomiting could trigger aspiration if neurological symptoms appear during the attempt.
The specific risks for a petite 3 kg frame
Smaller cats reach toxic blood concentrations faster. Your cat's size means even trace amounts circulate through her system more intensely than they would in a 5 kg cat. Vets calculate treatment doses by weight, so her petite stature makes precision timing even more crucial.
Watch for subtle changes: excessive licking of lips, unusual tail movements, or a "jumpy" reaction to touch. These often precede obvious tremors. Keep her in a dim, quiet space—bright lights or loud noises can accelerate seizure onset in permethrin cases. Have a carrier ready; if symptoms develop during transport, cover it with a blanket to minimize sensory input.
The step-by-step response, from the first minute to the vet visit, is in our Permethrin Toxicity in Cats emergency guide.