Lactose intolerance is gastrointestinal upset due to the inability to digest significant quantities of lactose. Lactose is a sugar found in milk and other dairy products.

Lactose intolerance is caused by a reduction in the digestive enzyme lactase. Lactase breaks down the sugar lactose into sugars that can be more easily absorbed. When not fully broken down, lactose ferments in the colon and causes symptoms.

Some people are born unable to make lactase. Others develop the intolerance over time.