Is it ok to give cow's milk to a cat?
by KittyKondo.com 

     Unfortunately, current research doesn't indicate whether cow's milk is actually harmful or beneficial to cats. For some cats milk will upset their stomach and cause diarrhea. This is because cats do not produce the digestive enzyme called 'lactase' necessary to digest 'lactose'(the sugar present in milk). Other cats will drink cow's milk from the time they are kittens and not have a single problem.

     When deciding whether to make milk a regular part of your cat's diet test whether your cat can take it. Give kitty the milk and if you don't notice him/her having an upset stomach or other litter problems shortly after giving the milk then it may be ok. There are also other milk products made for cats which are almost all lactose-free, such as Whiska's Cat Milk that your cat may enjoy.