Lactose intolerance happens when you have a shortage of the enzyme lactase. There are 3 main causes of lactose intolerance. Primary lactase deficiency-a gradual reduction in lactase production as you get older; congenital lactase deficiency-you have inherited a rare disorder where you produce little or no lactase; secondary lactose intolerance-when a separate condition damages the lining of your bowel and reduces the amount of lactase you produce. Diseases of your digestive system incl gastroenteritis, irritable bowel syndrome, coeliac disease and cancer or injuries can cause secondary lactose tolerance. It's usually only temporary and your symptoms should disappear within 2-4wks. However, if it's caused by a long-term condition, it may be permanent.
Attribution: BUPA