Some pets cannot tolerate milk because they do not produce an enzyme called lactase. Lactase helps with the digestion of lactose, which is found in milk. After weaning, the level of lactase activity falls to about 10 per cent of its peak activity. In some animals, diarrhea will occur if more lactose is consumed than the pet can digest. This is called "lactose intolerance".
The most common and overt signs of lactose intolerance are vomiting and diarrhea, which isn’t surprising because the digestive system isn’t working properly. Sometimes a dog will drink excess amounts of water because lactose intolerance can also cause dehydration.
It is not difficult to diagnose lactose intolerance in your dog. If the pet is being fed milk and dairy products on a regular basis, and the signs of lactose intolerance, like vomiting and diarrhea, are present, then it’s relatively easy to deduce that the pet is not tolerant of milk.