Does lactose intolerance cause diarrhoea?

Lactose is normally split into smaller sugars by lactase in the small intestine. When digestion is incomplete, lactose reaches the large bowel. It draws water into the bowel and bacteria ferment it, producing gas and changing stool consistency. Symptoms may begin after a dairy food and settle as the lactose passes through.

The response can depend on the type and amount of dairy, as well as an individual’s remaining lactase activity. Diarrhoea has many causes, including infection, medicines and other digestive conditions, so recurring loose stools deserve a proper assessment rather than self-diagnosis.

Record meals, dairy portions, stool changes and timing for several episodes. While arranging advice, choose fluids regularly and consider lactose-free alternatives. A clinician can guide a temporary lactose reduction and a structured reintroduction, which helps show whether lactose is contributing. Lactase products may help some people; ask a pharmacist about suitable use.

Do not remove every dairy food indefinitely without replacing its nutrients. If diarrhoea continues, focus on hydration and seek advice before making restrictive diet changes.

Seek medical advice for diarrhoea that persists or returns, or if it comes with weight loss, fever, night-time symptoms, ongoing pain or blood in the stool. Get urgent help for signs of dehydration such as very little urine, marked dizziness, confusion or inability to keep fluids down.

Does the timing after dairy support lactose intolerance? Should I have a breath test? How can I maintain calcium intake? Could infection, medicine or another bowel condition explain the diarrhoea?