Lactose, also called milk sugar, is a form of simple sugar found in dairy products. Some people are unable to digest lactose, and drinking milk or eating products containing lactose results in intestinal discomfort, gas, bloating, and other symptoms. Because lactose is not an important component of baked goods, lactose-free milk and other lactose-free dairy products can easily be substituted in most formulas. Other milk replacers, such as soy milk, can also be used.
Milk allergies are immune reactions to milk proteins rather than to lactose. This allergy is fairly common in infants, but most children grow out of it. The allergy is uncommon in adults. Those who suffer from it must usually avoid all dairy products.