What was the brand of the flea medicine?
What was the active ingredient?
What size dog was the flea medicine for?
Sorry, I got locked out from replying because it said someone else was replying. I think this was your second message.
Permethrin is very toxic to cats. The dose varies. Some cats are very sensitive and react to a drop of the dog flea preparations. Some cats tolerate more. Absorption through the skin is slower than if they can reach the spot and lick.
Clinical signs usually occur within a few hours, but may be delayed for up to 24 hours. The most common signs of toxicity are muscle tremors and seizures, hypersalivation, depression, vomiting and loss of appetite. This insecticide can cause death particularly if the cat has uncontrollable seizures. The trembling can be treated with methocarbamol.
If you are unable to get veterinary help tonight, continue keeping the cat cool and minimize exposure to anything that increases agitation(movement, light, noise). If he doesn't want to eat or drink, do not force him if it increases stress. If he is still having a reaction in the morning, get him to your vet as soon as you are able.
Pyriproxyfen (Nylar) is an insect growth regulator and is not toxic to cats.