Lactose intolerance is a condition caused by a decreased ability to digest lactose, a sugar found in dairy products. Those affected vary in the amount of lactose they can tolerate before symptoms develop. Symptoms may include abdominal pain, bloating, diarrhea, flatulence, and nausea. These symptoms typically start thirty minutes to two hours after eating or drinking milk-based food. Their severity typically depends on the amount a person eats or drinks. Lactose intolerance does not cause damage to the gastrointestinal tract.

Lactose intolerance is due to the lack of the enzyme lactase in the small intestines to break lactose down into glucose and galactose. There are four types: primary, secondary, developmental, and congenital. Primary lactose intolerance occurs as the amount of lactase declines as people age. Secondary lactose intolerance is due to injury to the small intestine. Such injury could be the result of infection, celiac disease, inflammatory bowel disease, or other diseases. Developmental lactose intolerance may occur in premature babies and usually improves over a short period of time. Congenital lactose intolerance is an extremely rare genetic disorder in which little or no lactase is made from birth. The onset of primary lactose intolerance, the most common type, is typically in late childhood or early adulthood, but prevalence increases with age.

Diagnosis may be confirmed if symptoms resolve following eliminating lactose from the diet. Other supporting tests include a hydrogen breath test and a stool acidity test. Other conditions that may produce similar symptoms include irritable bowel syndrome, celiac disease, and inflammatory bowel disease. Lactose intolerance is different from a milk allergy. Management is typically by decreasing the amount of lactose in the diet, taking lactase supplements, or treating the underlying disease. People are able to drink at least one cup of milk without developing symptoms, with greater amounts tolerated if drunk with a meal or throughout the day.

Worldwide, around 65% of adults are affected by lactose malabsorption. Other mammals usually lose the ability to digest lactose after weaning, and this was the ancestral state of all humans before the recent evolution of lactase persistence, which extends lactose tolerance into adulthood. Lactase persistence evolved in several populations independently, probably as an adaptation to the domestication of dairy animals around 10,000 years ago. Today the prevalence of lactose tolerance varies widely between regions and ethnic groups. The ability to digest lactose is most common in people of European descent, and to a lesser extent in some parts of the Middle East and Africa. Lactose intolerance is common among people of Jewish descent, as well as in many African countries and Arab countries. Traditional food cultures reflect local variations in tolerance and historically many societies have adapted to low levels of tolerance by making dairy products that contain less lactose than fresh milk. The medicalization of lactose intolerance as a disorder has been attributed to biases in research history, since most early studies were conducted amongst populations which are normally tolerant, as well as the cultural and economic importance of milk in countries such as the United States.