Flea liquid, the one you get from a vet, NOT the over the counter ones. Flea collars accomplish nothing effective and can be dangerous to the animal. Take it off the cat and get proper flea treatment for the cat too. I stopped using flea collars many many years ago, before there were flea drops to use, after finding that on my cat, under the collar, was a ring of red and weeping skin with all the hair gone. Dreadful things. Don't know why anyone buys them or why they're still sold.