Lactose intolerance (or hypolactasia) is the term for the normal decline in the level of lactase, an enzyme needed for proper metabolization of lactose (a sugar that is a constituent of milk and other dairy products), in human beings after weaning.
Lactose intolerance is normal in all adult mammals, including human beings. However, a few ethnic groups—particularly certain European groups—continue to produce lactase throughout their lives, and in these populations lactose intolerance is usually treated as a medical disorder.