Some people have cramping, gas, and diarrhea after eating or drinking milk products. This problem is called lactose intolerance. It signals a lack of lactase, the enzyme needed to digest lactose. Lactose is the sugar in milk. It is not easy to meet calcium needs without drinking milk. There are some nondairy foods with good amounts of calcium. An adult with lactose intolerance should aim to get the amount of calcium contained in 3 cups of milk daily.
Daly-Koziel, Kathy and Walters, Jackie, "Milk Group: When Dairy Is a Problem" (2006). Family and Consumer Sciences Publications. 34.