Some people who have trouble digesting milk have lactose intolerance, a condition in which the body is deficient in lactase, the enzyme necessary to digest lactose milk sugar. When lactose isn’t broken down, it passes into the large intestine, causing gas, diarrhea, and other unpleasant symptoms. These issues can often be avoided by consuming lactose-free dairy products, or by taking a Lactaid digestive enzyme.
Dairy allergies, on the other hand, are an immune response to a protein found in dairy products, rather than an enzyme deficiency, and they are much more common. If you have an allergy to cow’s milk, eating dairy products—even if they’re lactose-free—will continue to cause health problems, including diarrhea and abdominal cramping. But symptoms aren’t limited to digestive distress. Dairy allergies can also cause skin rashes, nasal congestion, joint aches, asthma, and earaches.