The National Institute of Child Health and Human Development wrote in their Jan. 2006 publication "Lactose Intolerance: Information for Health Care Providers":

“Individuals with lactose intolerance are unable to digest significant amounts of lactose due to an inadequate amount of the enzyme lactase.

Clinical symptoms of lactose intolerance can include abdominal pain, diarrhea, flatulence, and bloating. The severity of symptoms differs, often depending on the amount of lactase remaining in the body and how much lactose has been consumed.

Some patients with lactose intolerance may believe they are allergic to milk or milk products. A milk allergy, however, is related to the proteins in milk rather than the lactose.”

Jan. 2006