More than 50% of the world's population is lactose intolerant, which is what happens when the small intestine doesn't make enough of the enzyme lactase, leading to difficulties digesting the milk sugar lactose. Lactose intolerance can develop anytime from infancy to adulthood and it can be tricky to diagnose because the symptoms may mimic other conditions, especially irritable bowel syndrome (IBS) which can also cause diarrhea, gas, bloating, nausea and abdominal cramping.
When I see someone in my office with symptoms suggestive of lactose intolerance, I'll usually ask them to avoid any and all dairy for a period of two weeks to see if their symptoms improve. A lactose hydrogen breath test is a more formal way of diagnosing lactose intolerance but more cumbersome than empirical avoidance. In the setting of lactase deficiency, a test dose of lactose will pass undigested from the small intestine to the colon where it undergoes fermentation by bacteria into hydrogen and other gases. A rise in the level of these gases during the test is considered evidence of lactase deficiency.
Once the diagnosis is made, either empirically or with formal testing, some people choose to continue to eliminate dairy products from their diet as a way of controlling their symptoms. For those with milder symptoms or who consider the dietary change a hardship, I recommend reintroducing small amounts of yogurt and hard cheeses, which contain less lactose than things like ice cream and mozzarella. Most people with low lactase levels can tolerate small amounts of dairy but will have symptoms with larger doses.
One of the important things to keep in mind when treating people with lactose intolerance is that although it is exceedingly common, it can also be a sign of other problems in the gastrointestinal tract. Celiac disease, an allergy to gluten-containing foods such as wheat, rye and barley may result in accompanying lactose intolerance because of the damage to the villi of the small intestine. Likewise for Crohn's disease where inflammation to the lining of the small intestine may result in a loss of the lactase enzyme. Gastrointestinal infections such as Giardia and Rotavirus are a common cause of secondary lactose intolerance which can be temporary or permanent. In the setting of gastroenteritis I'll often counsel people to temporarily avoid dairy and then cautiously reintroduce it once the acute illness is over.
If you think you might be lactose intolerantm there is no problem with doing an avoidance trial, but if your symptoms don't resolve after eliminating dairy, then by all means seek medical advice to make sure you don't have an underlying gastrointestinal condition such as IBS, celiac disease, Crohn's or an infection.