Lactase refers to an enzyme whose main role is to break down the sugar found in milk called lactose. In humans, this enzyme is produced by the cells that line the small intestines. Most people are born with the ability to produce lactase because the body needs it to be able to digest milk. However, an estimated two-thirds of individuals make less of this enzyme as they grow older and may even totally stop producing it as they become adults. When this happens, adult-type hypolactasia which is commonly known as lactose intolerance occurs.

Some of the major factors which can determine if a person will develop lactose intolerance are genetics and ethnicity. There are also some rare cases wherein an individual has an inborn deficiency in lactase. This condition is called congenital absence, and results from the mutation of the gene, which is responsible for producing lactase. This inborn condition can be diagnosed easily after birth and can be alleviated or treated. Some diseases which affect the lining of the small intestines may also cause lactase deficiency because they may interfere with the ability of the cells to produce this enzyme.

Lactase supplements may come in the form of pills or liquids. Compared to liquid supplements, those which are in the form of pills can live in environments with higher levels of pH. Aside from being used in supplements, this enzyme also has commercial purposes. One of the most common commercial uses is for the reduction of lactose levels in different dairy products, such as in low-lactose milk. Lactase can also change the taste of dairy products, so this enzyme is included in some types of flavored milk and syrups that are dairy-based, in order to increase their sweetness. Aside from enhancing the levels of sweetness, it can also make dairy products last longer and less prone to damage from bacteria.