I used the Hartz flea and tick unknowing that it was different…leathal. Thankfully, I my cat went in to tremors and i recognized the problem and got him to the hostpital. $550.00 later, my cat came home. My other cat, did not have a reaction except both cat had burn spots on their necks. You know there is an obvious problem when you walk into a vet hospital and the receptionist looks across the counter and asks if you used hartz tick medicine on the cat. Sad that this type of product is on the market.