My partner (Nathan) works in a laboratory as an analytical chemist, at a natural pyrethrin extracting factory. According to him they the largest supplier of natural pyrethrins in the world. He says that natural pyrethrins are completely safe for animals and humans, and I used natural pyrethrin shampoo on my kitty when I first got her (she was flea ridden and had very thin falling-out fur and skin irritation from biting/scratching). My experience is it's very effective for killing fleas, the wash basin was full of dead and dying fleas. I noticed no ill side effects afterwards, and I lathered her twice with it including her face and ears to ensure I didn't miss any fleas (Nathan assured me it is safe to eyes at low concentration levels of pyrethrin contained in the shampoo).
Synthetic pyrethrins, including permethrin, ARE highly toxic to cats. Cats have a particular sensitivity to them, and they should never be used.
Eight days after flea shampooing kitty, her fur is thick and luxuriant, and she hasn't scratched or bitten herself at all that I've seen. I find that natural pyrethrin flea shampoo is effective, safe, and I would recommend it to others.