A few weeks ago I got a Hartz flea collar for one of my cats. It seemed to be working fine. Then a few days ago when I was carrying him I saw red in his fit where the collar was. I immediately took it off and checked. About an one inch section of his neck was raw. Now for the other cat. You may have noticed that I only bought the collar for 1 cat. The reason for that is in the past when I would get her a collar, she would act goofy and out of character. We thought it was a bad reaction to the flea medicine in general, now looking back I think it may have been some thing more. I’m taking my car to the vet tomorrow to get him checked out.