Cats don't have the metabolic pathways that dogs and humans have that process and neutralize many common insecticides belonging to a class called pyrethroids. For cat flea control meds, these are substituted with fipronil, which is (less) toxic and doesn't get absorbed through the skin, though when we were dealing with a flea infestation a few years back we still had problems when one of our cats ended up being flexible enough to scrape the gel off its neck and lick it off its paw. Bottom line, though, cats tend to have a lot more trouble with metabolizing medications generally, and tend to encounter problems with "cat safe" meds more often than you'd expect. You have to be careful about monitoring your cat after starting a medication generally.