I have to give my CKD cat subcutaneous fluids. At first I had the help of a neighbor who is vet tech trained. But, I had to figure out a way to do this by myself as my cat is not the most docile. Bring on the Cat Sack. He is 11lbs and the large fits him, though if he were a longer cat, I'd want to size up. It holds him securely so he can't get away and allows me to pet his head while he gets his fluids. I undo the zipper on the top as far as needed to administer the fluids.