After consuming milk, people with congenital lactase deficiency may experience symptoms of nausea, vomiting, and abdominal cramping. Congenital lactase deficiency is usually diagnosed in infancy, and babies with the condition may be diagnosed with a failure to thrive.
Congenital lactase deficiency is different than a true milk allergy. In people who have a milk allergy, it is the protein in milk that causes an allergic reaction, resulting in symptoms such as hives, eczema, nausea, vomiting, and diarrhea.
Neither is congenital lactase deficiency the same as acquired lactase deficiency, which usually develops over time, typically starting after a person is about 2 years old. After the age of 2 years, some people, especially those of Asian or African descent, start losing the ability to produce lactase -- sometimes completely.
Either way -- whether congenital or acquired -- a lactase deficiency causes lactose intolerance. People with lactose intolerance may experience symptoms of nausea, vomiting, or diarrhea after ingesting milk products.