Lactose intolerance means you have trouble digesting lactose. Lactose is the sugar found in milk and foods made with milk. The small intestine needs lactase enzyme to break down lactose. With lactose intolerance, you may not feel well when you eat or drink something with lactose because you don't have enough lactase enzyme. Many people have problems digesting lactose. Some people become lactose intolerant as children. In others, the problem starts when they are teenagers or adults. Lactose intolerance is rare in babies. Premature babies may be lactose intolerant for a short time after they are born. This publication by the National Institutes of Health (Publication No. 10-2751) provides information about the symptoms, diagnosis and treatment and management of lactose intolerance.