For most people, lactose intolerance develops naturally as they grow older. The small intestine often begins to produce less lactase after age two. Certain digestive diseases such as Crohn's disease, Celiac disease (a digestive disease that damages the small intestine and interferes with absorption of nutrients from food), infections, and injuries to the small intestine can also reduce the amount of lactase available to process lactose properly.
The most common foods that are high in lactose include dairy products such as milk, ice cream, and cheese. Lactose is also added to some foods, such as bread and baked goods, cereals, salad dressings, candies, and snacks.
Foods that contain whey, curds, milk by-products, dry milk solids, and nonfat dry milk also contain lactose.
Lactose is also present in about 20% of prescription medications, such as birth control pills (oral contraceptives), and about 6% of over-the-counter medications, such as some tablets for stomach acid and gas.