The most common question veterinarians hear from cat owners is, "Is it OK to give my cat milk?" It's almost always followed by, "I only give her a little bit in the morning and she doesn't seem to have a problem with it. She really likes it."
Some cats develop milk or lactose intolerance and experience gastrointestinal upset (vomiting, diarrhea) when allowed to imbibe nature's most obvious cat food.
If you share your home with one of these cats, save the milk for the human kids; if Fluffy enjoys his liquid snack and has no reactions including weight gain feel free to indulge him. Skim milk is preferable for overweight cats. Never give chocolate or sour milk, and do not offer milk if your cat has food allergies. Remember, too much of a good thing is not a good thing anymore.