Lactose is a major type of sugar found in milk and milk products, including human milk. Lactose makes up less than eight percent of the solids in milk. Lactose is not found naturally in any other food aside from dairy products.

Lactose intolerance is caused by the inability of the body to break down milk sugars. In a normal person, lactase, an enzyme produced by the small intestine, breaks down lactose so it can be absorbed into the bloodstream. People who are lactose intolerant cannot produce enough lactase and are unable to process lactose.

As a rule, mammals lose the ability to digest lactose as they get older. Humans have undergone a mutation that allows them to retain a 10 percent capacity to produce lactase. This is normally enough to allow humans to digest milk and dairy products. Some people, however, don't retain this 10 percent, so they cannot consume any fresh milk and milk products at all throughout their adult lives.

Lactose is often added to prepared foods to prevent caking or as a coating. Since lactose is almost tasteless, it also makes for an ideal filler in bread and other baked goods, including cereals, pancakes, and cookies. Lactose is also commonly added to canned and frozen vegetables to prevent discoloration. All commercial lactose used in western countries is obtained from cow's milk. In the Middle East and some African countries, commercial lactose is sometimes a by-product of camel or goat's milk.

Hidden lactose is also found in many ready-to-eat powdered products such as dehydrated potatoes, soups, and meal-replacement supplements. Foods that are labeled non-dairy, including coffee creamers, can also contain lactose in the form of whey or dry milk solids.

Lactose is used in hundreds of over the counter and prescription drugs as a filler or coating. This include hundreds of pills, such as chewable Pepcid AC, birth control pills, and throat lozenges. While the amount of lactose present in a single pill is hardly enough to affect most people, those with severe allergies should talk to their doctors before starting any new medication. Since lactose is essentially a sugar, people who suffer from diabetes should also limit or control its intake.