There are two ways lactose intolerance can develop. It can be an inherited condition (which, rarely, may be present at birth, but usually shows up during childhood or later), or it can be triggered off by illness – which affect the digestive system.
Lactase is a very sensitive enzyme and is easily affected by such illness. It usually returns to former levels after the person has recovered - but sometimes not for quite a while - so the lactose intolerance is only temporary.
Occasionally, however, the affected part of the intestines may be more severely damaged and intolerance to lactose will be permanent.
The following conditions may cause lactose intolerance:
Irritable bowel syndrome
Sensitivity to gluten in wheat, etc
Vitamin B2 (riboflavin)
Certain tropical diseases - e.g. cholera, sprue
Temporary lactose intolerance can also be caused by certain medicines, some antibiotics, and those drugs used to treat arthritis, can temporarily upset the digestive system and reduce levels of the lactase enzyme.