Lactose intolerance, also called lactase deficiency, means you aren't able to fully digest the milk sugar (lactose) in dairy products. It's usually not dangerous, but symptoms of lactose intolerance can be uncomfortable.
A deficiency of lactase — an enzyme produced by the lining of your small intestine — is usually responsible for lactose intolerance. Many people have low levels of lactase, but only those who also have associated signs and symptoms have, by definition, lactose intolerance.
You can control symptoms of lactose intolerance by carefully choosing a diet that limits dairy products.