Once kittens are weaned, they usually don’t get milk anymore. During the further development of the gastrointestinal tract, the ability to digest milk sugar decreases. Eventually, the enzyme that takes care of that digestion will disappear completely. Adult cats can suffer from diarrhea if they drink regular (cow) milk. If you still want to give milk now and then, then special cat milk is a good alternative. This milk is low in lactose (milk sugar). Note that a cat doesn’t have to drink milk to stay healthy. Think of it as an extra.