Lactose is a type of sugar that's in milk and dairy products.
Lactase is an enzyme in your small intestines that digests lactose so your body can absorb it.
Lactose intolerance happens when you can't digest lactose. That's because your small intestine doesn't make enough of the enzyme lactase, which digests lactose. A food "intolerance" means you can't tolerate eating certain food because you can't digest it.
Being allergic to cow's milk is not the same thing as being lactose intolerant. People with cow's milk allergy can digest milk properly, but proteins in the milk trigger an allergic reaction.
All infants have lactase in their intestines so they can digest their mother's milk. Nearly everyone makes less lactase as they grow up. Once the amount of lactase is no longer enough, people start to get symptoms of lactose intolerance after eating dairy products. However, people whose families come from Northern Europe are an exception. Many people from this area can digest dairy products their whole life.
Lactose intolerance can also start after you have an injury or surgery on your intestines or have another bowel problem.
Lactose intolerance may cause:
You usually have to drink at least 8 to 12 ounces (250 to 375 milliliters) of milk to get symptoms. If you get symptoms after only a few sips of milk, it's probably not from lactose intolerance.
Doctors will have you stop eating or drinking dairy for a while. If your symptoms stop and then return when you eat dairy again, you probably have lactose intolerance.
Other tests are rarely needed unless:
Sometimes, doctors will do a breath test. They have you drink a lactose drink and test the air you breathe out. This can show how well the lactose was digested.
You should stop eating the foods that cause your symptoms. Yogurt and cheese have less lactose in them and may not cause symptoms. You can get special milk that has less lactose in it.
You also can take lactase pills to break down the lactose when you plan to eat or drink milk or dairy.
People who must avoid dairy products may need calcium supplements to prevent calcium deficiency.