I had made the mistake of buying Hartz 3 in 1 Flea-Control Collar for Cats. It's all good in theory: an adjustable collar treated with an anti-flea solution that will protect the cat for 7 months. In reality though, it does not work. My cats wore these things and kept scratching. I thought it might take some time to see the results, but time went by, and fleas persisted. Perhaps some of them were affected, and the cats got some relief from biting, but the problem was not solved.
Later on, I had learned that I had gotten lucky: my cats suffered no damage from the collar. Others often do. The chemicals in the collar are dangerous, especially if the cat starts chewing on it. That can actually kill them. My vet, who had directed me to an excellent-working and safe flea solution, had also said that numerous cats have been hurt by Hartz flea control products, particularly the collar. I wonder why it is not pulled off the market; it should be.