Cats should not drink cow's milk. Most cats are lactose intolerant, meaning they lack the enzyme lactase needed to digest lactose, the sugar in milk. Drinking cow's milk can lead to vomiting, diarrhea, stomach pain, and other digestive issues for cats. It can also contribute to obesity due to the high fat content. While kittens need their mother's milk early on, adult cats do not require milk in their diet and are best served a balanced cat food diet with water. Special lactose-free or formulated cat milk can be given rarely but is not necessary or particularly beneficial for cats' health.