Soon after i applied hartz one spot to my cat, he seemed to be irritated in the spot i applied it, so i came online to look for answers. this site popped up before anything else, so i got scared out of using hartz one spot. since then i have researched the main ingredient methoprene. when researching it, i found the e.p.a report that investigates the deaths apparently caused by its flea products. the majority of deaths involved the cat getting exposure to the product in this line for dogs, not cats. there is some controversy as to whether there was a product mix up at the company or not. the other deaths involved an allergic reaction to the product, and there are a couple of deaths that have not been substantiated either way. anyway the point is, read the freakin label!!!! dont use a dog flea killer on a cat, theyre designed specifically for either or, not both. if you kill your cat cause your too dumb to read the label, dont blame the company!!! also, just like people, you arent going to know youre allergic to something till you try it. how is hartz supposed to prevent allergies? and how are they going to force you to use the product properly? or the right product? the highest incidence of problems with this product, have been skin hypersensitivities, where the cat acts kinda bonkers, cause its trying to get it off. this happened to my cat, but i no longer blame the product, i blame my little guys sensitive skin.i wont be using one spot for him, but i also wont discourage anyone else from using it either, its probably great for lots of cats, just not mine. From now on, i wont be taking peoples ignorant, uneducated opinions for fact, i’m going to look for solid investigative facts. theres also a study done by the national animal poison control center, that found similar results to the e.p.a’s. care about your pet enough to READ THE LABELS!!!!!!