What is Lactose Intolerance? Lactose intolerance (clinically known as Lactase Deficiency ) means your digestive system is unable to fully break down and digest lactose, the primary sugar found in milk and dairy products. It is caused by a deficiency of lactase —a crucial enzyme produced by the lining of your small intestine. While the condition is usually not dangerous, the resulting gastrointestinal symptoms can be highly uncomfortable. It is important to note that lactose intolerance is entirely different from a milk allergy, which is a severe immune system response to dairy proteins. Symptoms Signs and symptoms typically begin 30 minutes to 2 hours after eating or drinking foods that contain lactose. The severity often depends on the amount of lactose consumed and your body's remaining lactase levels. Common symptoms include: Diarrhea or loose stools. Nausea and, in rare, severe cases, vomiting. Pai...