What should you know about lactose intolerance sudden onset?

Lactose is the natural sugar in milk. The small intestine uses lactase to break it down; when less lactase reaches the gut, undigested lactose draws in water and is fermented by bacteria. This can cause bloating, cramps, gas or diarrhoea after dairy. A stomach infection, inflammation or another digestive condition can temporarily reduce lactose digestion. A new pattern is not proof of lactose intolerance, because food allergy, coeliac disease, irritable bowel syndrome and infection can cause similar symptoms. See the guide to [Lactose Intolerance](/conditions/lactose-intolerance/).

Keep a brief food-and-symptom diary, noting the dairy food, portion, timing and symptoms. For a short period, reduce obvious lactose sources while choosing nourishing alternatives; yoghurt and hard cheese may be easier for some people than milk. Check labels on sauces, baked foods and medicines, and avoid removing all dairy for a long time without planning calcium and vitamin D sources. A clinician may suggest a structured elimination and reintroduction or [Food Intolerance Testing](/treatments/diagnostics-and-screening/food-intolerance-testing/). [Nutrition Counselling](/treatments/lifestyle-and-wellness/nutrition-counselling/) can help protect dietary balance.

Arrange medical advice if symptoms continue after the suspected trigger is removed, keep returning, or lead to weight loss, poor intake or night-time disturbance. Seek urgent help for facial swelling, wheezing, faintness, blood in stool, severe persistent abdominal pain, repeated vomiting or signs of dehydration such as very little urine and marked dizziness. For symptom context, see [Abdominal Pain](/symptoms/abdominal-pain/), [Bloating](/symptoms/bloating/) and [Diarrhoea](/symptoms/diarrhoea/).

Ask whether a recent infection, medication or bowel condition could explain the change; whether a supervised lactose challenge is suitable; and how to meet calcium and vitamin D needs while symptoms are investigated.