When to worry about diarrhoea with lactose intolerance

Lactose intolerance means the small intestine does not digest lactose efficiently. Undigested lactose draws water into the bowel and is fermented by gut bacteria, which can cause diarrhoea, cramps, bloating and wind after dairy foods. Symptoms usually follow a food pattern, so diarrhoea that occurs without lactose exposure or continues despite avoiding a suspected trigger needs a fresh assessment.

Record what you eat and when symptoms begin. Choose lactose-free alternatives or smaller portions if these have been advised, and check that your overall diet still supplies enough calcium and other nutrients. Do not remove every dairy product indefinitely without discussing it with a clinician or dietitian. Replace lost fluids when stools are loose.

Get urgent help for severe abdominal pain, faintness, confusion, inability to keep fluids down, very little urine, blood or black stool. Arrange a review for weight loss, persistent diarrhoea, symptoms that wake you at night, fever, repeated vomiting, or reactions involving swelling, wheezing or breathing difficulty, which may indicate allergy rather than intolerance.

Ask whether your pattern fits lactose intolerance, whether another bowel condition should be checked, and how to test food triggers safely. Mention your usual dairy intake, timing of symptoms, weight changes, medicines and any skin or breathing symptoms.