Longstanding diarrhoea needs to be checked out by your doctor as it is important to find out why this is the case and if your child is affected by it in any way. Some of the causes for longer duration diarrhoeas are: Lactose intolerance (cow’s milk sugar intolerance), Cow’s milk protein allergy, wheat intolerance or allergy, Coeliac disease (Gluten intolerance leading to malabsorption), Cystic fibrosis, inflammatory bowel disease (such as Crohn�s disease or ulcerative colitis), irritable bowel syndrome etc