All of my cats are indoor only and are microchipped and they all wear the saftey break away collars except for Sunny. Sunny became so stressed out that he made himself ill. His bladder became inflammed and he kept jumping in and out of the box and was barely able to pee. The vet didn't find any crystals or infection and he said that it could've been caused by stress. I put two and two together and the only change was the collar. I took it off of him and he's never had any more problems. I guess some cats just aren't meant to wear collars. Good luck.