Side effects of lactose intolerance

Lactose is the natural sugar in milk and some dairy products. When the gut has too little lactase, the enzyme that breaks lactose down, undigested lactose passes into the bowel. It draws in water and is fermented by gut bacteria, producing gas and changing bowel movements. Symptoms may start after a meal and can vary with the amount eaten and the type of product. Lactose intolerance is different from a milk allergy, which can involve skin or breathing symptoms. Read about [Lactose Intolerance](/conditions/lactose-intolerance/), [Bloating](/symptoms/bloating/) and [Diarrhoea](/symptoms/diarrhoea/).

Keep a food and symptom diary so a clinician can look for a repeatable pattern. Some people tolerate small portions, yoghurt or hard cheese better than milk; others need lactose-free alternatives. Avoid cutting out all dairy for a long time without planning calcium and vitamin intake. A clinician may discuss [Food Intolerance Testing](/treatments/diagnostics-and-screening/food-intolerance-testing/) and a supervised dietary trial. [Nutrition Counselling](/treatments/lifestyle-and-wellness/nutrition-counselling/) can help you choose suitable substitutes. Persistent symptoms deserve review because infections, coeliac disease and bowel conditions can look similar.

Arrange assessment for ongoing diarrhoea, abdominal pain, weight loss, blood in the stool, fever, night-time symptoms or signs of poor nutrition. Seek urgent help for severe abdominal pain, repeated vomiting, fainting or swelling of the lips or tongue with breathing difficulty. Those allergy-type symptoms are not typical of lactose intolerance. Sudden watery diarrhoea can also cause dehydration, especially when drinking is difficult.

Ask whether your symptoms fit lactose intolerance or another condition, how to test the link with dairy safely, which foods need replacement, and how to maintain calcium and vitamin intake. Ask when symptoms should improve after a planned dietary change.