As the others have said, in normal cats, cat milk is a food and not a drink, so they still need water, and might also need their food intake reducing to compensate. I use it a lot when I get thin fosters in, to help build them up, and lactating queens can benefit a lot from KMR, or cat milk (KMR is probably the better option, but I Find cat milk easier to use, cos it is ready mixed). You can't give the adult cat milk to kittens under the age of 6 weeks though, so watch them for investigating it when they start showing an interest in food - you can get kitten milk from Asda though.