Most cats are lactose intolerant.

Though cartoon depictions of cats happily drinking it from a saucer suggest otherwise, milk isn’t actually good for cats. In fact, most cats are lactose intolerant, a condition that develops after kittens stop nursing, sometime between 6 and 12 weeks of age. Although kittens produce lactase, an enzyme that breaks down lactose, their bodies stop producing it once they’ve been weaned — which helps explain both why they’re still drawn to milk and why it’s no longer good for them. 

Most cats can drink a small amount of milk without experiencing gastrointestinal distress, but it provides no nutritional benefits to adult felines. And while there are some lactose-free alternatives made for cats, they’re intended as occasional treats (rather than a staple of their diets. Water, though perhaps less exciting, is all your kitty needs to stay hydrated.