I started using a flea collar as I was going to board my mini for a few days; it was required. She became very lethargic and quiet after a week or two. I thought is was medicine she had been on previously, at first. However, I took the flea collar off and by the next morning she was much better. Fine, in a few days. It was one of the Bayer ones, Seresto. Perhaps others are aware of this possible reaction, but thought I would mention it. Net research indicates reactions are much more common than initially suggested.