Milk is not good for dogs. Once dogs are weaned, their bodies cannot properly digest milk. Drinking milk can upset dogs' digestive tracts and give them diarrhea.
Although a dog may wag his tail the whole time he is lapping up milk, you and he will both be sorry later. That is because a dog's digestive tract does not have the enzymes required to break down lactose. Feeding your dog any milk-based product can lead to the same result experienced by a lactose-intolerant person who consumes a milkshake. If your dog does ingest some milk and shows signs of diarrhea or other distress, contact your veterinarian for advice.