Contrary to popular belief, adult cats should not drink milk but water! It is possible to give them milk occasionally to please them as a treat. Kittens are fed with their mother's milk or formula, cow's milk does not cover their needs.
Milk: feeding cats before weaning
Kittens are fed from birth to weaning with their mother's milk. Kibbles and pate are gradually introduced from the age of 3 weeks so that the kittens are weaned around the age of 7 weeks.
After weaning, cats no longer need milk and drink water.
If it's necessary to prepare bottles, then it's important to choose a formula that's specially designed for kittens. This will provide the kittens with everything they need to grow. However, you should not give them cow's milk, which is too rich in lactose and does not cover the kittens' protein and energy needs.
And in adulthood?
Kittens are able to digest their mother's milk thanks to an enzyme found in the small intestine: lactase.
The activity of this enzyme decreases from the age of 6 weeks, kittens progressively lose their ability to digest milk as they grow up.
This makes it very difficult for adult cats to digest milk. Undigested lactose can cause intestinal fermentation and diarrhea.
Therefore, it is important to remember that water should be the main drink for adult cats. Occasionally, cats can be given small amounts of milk as a treat. Cats are able to tolerate 10ml of milk/kg, so you can give your cat up to 40ml of milk from time to time if it weighs 4kg.
It is best to choose a low-lactose milk to facilitate the digestion of cats. For example, there is Whiskas Catmilk low in lactose that can be given to cats as a treat.