We asked this from our vet when we got Casper and Kitty, and he said we can give milk that is made for cats, but don't give milk which is made for humans. Cows milk isn't good for cats, dunno why, but that's what our vet said. We gave Casper and Kitty milk for about 2 months (they also had access to water), and they then themselves switched to water and we stopped giving milk. They seem to prefer water better.