Dogs and cats do not have the proper enzymes to properly break down the sugar in milk which is called 'lactose.' You may have heard of people who are lactose-intolerant. They are also missing these digestive enzymes. If the proper enzymes are not present, the lactose remains undigested and tends to ferment in the intestine and cause diarrhea. Some pets can tolerate a little milk, others, none at all. If your pet enjoys and appears to tolerate milk, you can give your pet small amounts. Better yet, give your pet one of the special cat or dog milks on the market that have had the lactose removed.