It was mildly annoying when they were just making patches where we could see their white skin where black hair should be. We worried about hairballs. But mostly we tried to gently distract them when we caught them. We had hope they'd get past it, like another of our cats who stripped the hair off his tail for about a year but then quit. One of our boys now has a raw patch though and I don't know what to do for him. It's in an awkward place - hard to cover - and we can't watch him every minute.