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".
Lactose intolerance is a condition that occurs when the body is unable to digest a sugar, known as lactose, which is naturally found in milk and dairy products. Cats often develop symptoms of lactose intolerance when they eat dairy products, but these symptoms are usually due to the type of digestive system that a cat has rather then an actual lactose intolerance condition.
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 cat will drink excess amounts of water because lactose intolerance can also cause dehydration.