I bought a Hartz cat flea collar for my 14 month old cat & she immediately began acting weird and lethargic. She wore it for a day or so, and then I took it off of her. She then gradually stopped eating until about a week later she completely stopped eating. She was admitted the hospital and a battery of tests was done. The only thing that could find was an inflamed pancreas and that there was something in her stomach that wasn’t digesting. She is doing better now, but I feel that it was either the flea collar (thank God I took it off after only a day or so) or clumping litter (which I didnt know was a problem either until I did research when she got sick) At first I thought it was definitely the litter, but then I stumbled onto this site and because of the slightly inflamed pancreas, I am starting to think it might have been the flea collar and the inflamed pancreas is what stopped her normal digestion….This cost me $2000 to find out and she probably has no permanent damage, but I’m sure she would have if I had not acted quickly and took her to more than one vet to get to the bottom of what was wrong with her!