What causes lactose intolerance?

    Written by:
    Natalka Roshak

    Fact-checked

    by:

    Shyam Jayaraman, Louie Neri
    Last Updated: May 16, 2025

    Lactose intolerance can be classified into 3 types: primary, secondary, and congenital. Primary lactose intolerance (the most common type) results from a natural decline in lactase production, secondary lactose intolerance occurs due to damage to the small intestine, and congenital lactase deficiency is a rare genetic condition that is present from birth.

    There are three types of lactose intolerance: primary, secondary, and congenital.[1]

    • Primary lactose intolerance is the most common. This is the normal drop in intestinal lactase production that occurs for most people between ages two and five.
    • Secondary lactose intolerance is a sudden decrease in lactase production due to damage to the small intestine. This is usually a transient condition resulting from conditions like infection, celiac disease, Crohn’s disease, food allergy, or small intestinal bacterial overgrowth (SIBO).[1]
    • Congenital lactase deficiency (CLD) refers to an inability to produce lactase from birth. It’s an autosomal-recessive disorder, caused by a mutation in the LCT gene, and it is extremely rare.[2]