Flea collars can cause a severe, local reaction because they carry a high quantity of flea killer in a small area. They are of extremely limited effect also. I used to use them before better options became available, but I do recall a cat coming into my vet with what was obviously a severe reaction to the flea collar. The vet (a locum) He also got terribly stroppy with me because I suggested (out of eashot of the client) what I felt the problem might be, and it offended his sense of self-importance, but that is another story) , he misdiagnosed the problem, having discounted the collar because it was no longer there, and prescribed meds that did not deal with the problem. Fortunately next time the cat came in it was properly diagnosed. They are not something I would use whilever there are more appropriate treatments available, like Frontline and Advantage.
