Feeling bloated, uncomfortable or suddenly needing the toilet after a milky coffee can make lactose intolerance seem obvious. But digestive symptoms after dairy are not always caused by lactose, and being lactose intolerant does not necessarily mean you have to stop eating every dairy product.
Lactose intolerance happens when the small intestine does not produce enough lactase, the enzyme needed to digest lactose — the natural sugar found in milk.
When lactose is not properly absorbed in the small intestine, it passes into the large bowel. Bacteria then ferment it, producing gas and drawing water into the bowel. This can cause bloating, cramps, wind and diarrhoea.
The amount a person can tolerate varies enormously. Someone may feel perfectly well after a little milk in tea but develop symptoms after a large milkshake. Another person may tolerate hard cheese and yoghurt but struggle with ordinary milk.
This guide explains the typical symptoms of lactose intolerance, how it is diagnosed, how hydrogen breath testing works and how to change your diet without unnecessarily removing nutritious foods.
What Is Lactose Intolerance?
Lactose is a sugar naturally present in the milk of mammals, including cows, goats and sheep.
To absorb lactose efficiently, the small intestine produces an enzyme called lactase. Lactase breaks lactose into simpler sugars that can then be absorbed.
If there is not enough lactase, some lactose remains undigested and travels into the colon.
There, gut bacteria ferment it.
This produces gases and other compounds and can increase the amount of water inside the bowel, leading to symptoms such as:
- bloating;
- abdominal cramps;
- excess wind;
- rumbling;
- diarrhoea;
- nausea.
The NHS describes lactose intolerance as a digestive problem rather than an allergy. Symptoms can begin within minutes or several hours after eating or drinking something containing lactose.
If bloating is your main symptom and dairy does not consistently trigger it, our guide to bloating, common causes and when to worry covers several other possibilities.
What Does Lactose Intolerance Feel Like?
The classic pattern is digestive symptoms developing after foods containing a meaningful amount of lactose.
You might notice:
- abdominal bloating or swelling;
- cramping or stomach pain;
- gurgling or rumbling;
- increased wind;
- loose stools or diarrhoea;
- urgency to open your bowels;
- nausea.
The severity depends partly on how much lactose you consume and how much lactase activity remains in your bowel.
This dose effect is useful.
For example, someone may have:
A splash of milk in tea — no problem.
A small yoghurt — perhaps no problem.
A large latte followed by ice cream — bloating, cramps and diarrhoea.
That pattern is quite different from a food allergy, where even a relatively small amount of the food can potentially trigger an immune reaction.
Symptoms also overlap heavily with other digestive conditions, particularly irritable bowel syndrome (IBS).
That overlap is one reason repeatedly diagnosing yourself by cutting out foods can become confusing.
Lactose Intolerance Is Not the Same as Milk Allergy
This distinction is particularly important in children.
Lactose intolerance is a problem digesting the sugar in milk.
Cow's milk allergy is an immune reaction to proteins in cow's milk.
They therefore require different approaches.
Lactose intolerance mainly causes digestive symptoms and does not cause anaphylaxis.
Milk allergy can cause symptoms such as:
- hives or an itchy rash;
- swelling of the lips, face or eyes;
- wheezing or breathing difficulty;
- vomiting;
- digestive symptoms;
- in severe cases, anaphylaxis.
The NHS specifically warns that food allergy can cause breathing difficulty, itchy skin and facial swelling, whereas lactose intolerance is a digestive intolerance.
This also explains why a product labelled lactose-free is not necessarily suitable for somebody with cow's milk protein allergy.
Lactose-free cow's milk still contains milk proteins.
Why Does Lactose Intolerance Develop?
For many adults, lactose intolerance develops because lactase production naturally decreases after childhood.
The ability to continue digesting large amounts of lactose throughout adulthood varies significantly between individuals and populations.
But lactose intolerance can also develop because the lining of the small intestine has been temporarily or permanently affected.
Primary lactose intolerance
This is the common form where lactase activity gradually becomes lower with age.
Symptoms may not become obvious until adolescence or adulthood.
Secondary lactose intolerance
Sometimes another condition damages the part of the small intestine that produces lactase.
Possible causes include:
- coeliac disease;
- inflammatory bowel disease;
- some bowel infections;
- bowel surgery;
- other injuries or inflammation affecting the small intestine.
The NHS confirms that lactose intolerance can occur secondary to conditions such as coeliac disease or inflammatory bowel disease and may improve when the underlying condition is treated.
This is an important reason not to assume that newly developing lactose intolerance is always the final diagnosis.
For example, untreated coeliac disease can damage the small-intestinal lining and reduce lactase activity. Once the intestine heals on a gluten-free diet, lactose tolerance may improve.
How Is Lactose Intolerance Diagnosed?
There is no need for everyone with suspected lactose intolerance to undergo extensive testing.
A GP may start by asking about:
- which foods trigger symptoms;
- how quickly symptoms begin;
- how much dairy causes problems;
- whether symptoms happen without dairy;
- changes in bowel habit;
- weight loss;
- family history;
- other medical conditions.
One practical approach is a short lactose elimination followed by reintroduction.
You temporarily reduce or remove lactose and see whether symptoms improve. Lactose is then deliberately reintroduced to see whether symptoms return.
This is more informative than permanently cutting dairy out after one bad stomach episode.
The NHS lists elimination followed by reintroduction among the approaches used when investigating food intolerance.
Blood tests may also be used when doctors are looking for another cause of symptoms, such as coeliac disease or inflammation.
If symptoms are persistent or the diagnosis remains uncertain, a hydrogen breath test may be considered.
How Does a Lactose Hydrogen Breath Test Work?
A hydrogen breath test assesses what happens after you drink a measured amount of lactose.
Normally, properly absorbed lactose should be broken down in the small intestine.
If lactose passes into the colon undigested, bacteria ferment it and produce hydrogen and other gases.
Some of that hydrogen is absorbed into the bloodstream, travels to the lungs and is breathed out.
The test therefore measures hydrogen in your breath at intervals after you drink a lactose solution.
A typical appointment involves:
- providing a baseline breath sample;
- drinking a lactose-containing solution;
- providing repeated breath samples over the next few hours;
- recording any symptoms that develop.
Chelsea and Westminster Hospital describes its lactose breath test as painless and typically lasting around two to three hours.
Preparation matters because food, antibiotics, smoking and other factors can affect gut bacteria and alter the result. Follow the specific instructions given by the testing service rather than relying on generic online preparation advice.
Breath tests also need to be interpreted in context. Other digestive conditions can sometimes influence results.
What Should You Eat If You Are Lactose Intolerant?
The aim is usually to find the amount of lactose you personally tolerate, rather than automatically creating a completely dairy-free diet.
The NHS advises that many people can prevent symptoms simply by reducing their lactose intake or having smaller portions.
Different dairy foods contain very different amounts of lactose.
Milk
Ordinary cow's milk contains lactose and is often one of the more obvious triggers, particularly in larger portions.
Lactose-free cow's milk is nutritionally similar but has lactase added so that the lactose has already been broken down.
This can be a useful option if you want to retain dairy without the lactose.
Hard cheese
Many mature hard cheeses contain very little lactose because much of it is removed with the whey and the remainder is reduced during maturation.
This means people who cannot tolerate a glass of milk may still manage foods such as mature cheddar or Parmesan without symptoms.
Yoghurt
Some people tolerate yoghurt better than milk because the bacterial cultures can help break down some lactose.
Tolerance varies by product and person.
Butter and cream
These can contain smaller amounts of lactose than milk, although portion size still matters.
Hidden lactose
Lactose can also appear in foods where you may not immediately expect it, including:
- some breads and baked products;
- breakfast cereals;
- instant mashed potato;
- soups and sauces;
- milk chocolate;
- protein shakes;
- some processed foods.
If you are very sensitive, checking ingredient labels can help identify patterns.
How Do You Avoid Calcium or Vitamin Deficiency?
Dairy products are important sources of calcium, protein and several vitamins.
If you simply remove milk, yoghurt and cheese without replacing those nutrients, your diet can become less balanced.
This is particularly important for children, teenagers, pregnant people and anyone at increased risk of osteoporosis.
Lactose-free dairy products can make this much easier because they usually retain the calcium and protein found in ordinary dairy.
If you prefer plant-based alternatives, look for products fortified with calcium and ideally vitamin D.
Not every plant drink has the same nutritional profile.
The NHS notes that people who remove dairy may be at risk of inadequate calcium intake and recommends calcium-fortified alternatives where appropriate.
Good non-dairy calcium sources can also include:
- calcium-set tofu;
- fortified plant drinks;
- fortified plant yoghurts;
- tinned sardines or salmon with edible bones;
- some leafy green vegetables;
- sesame and tahini.
If you are excluding several food groups or planning a restrictive diet for a child, getting advice from a dietitian is sensible.
Can Lactase Tablets Help?
Yes, for some people.
Lactase supplements provide the enzyme needed to digest lactose and are usually taken shortly before eating lactose-containing food.
They can be helpful when eating away from home or when you want to occasionally eat a food that normally causes symptoms.
The NHS notes that lactase supplements may reduce or prevent symptoms for some people.
They are not guaranteed to work perfectly.
Effectiveness depends on the dose of lactase, how much lactose you eat and individual digestive differences.
Lactase supplements also will not help if your symptoms are actually caused by something else — for example milk protein allergy, IBS or another food component.
When Might It Be Something Other Than Lactose Intolerance?
Bloating and diarrhoea are extremely common symptoms and have many possible causes.
If symptoms continue despite reducing lactose, another diagnosis should be considered.
Possibilities include:
- irritable bowel syndrome;
- coeliac disease;
- inflammatory bowel disease;
- other food intolerances;
- small intestinal bacterial overgrowth;
- persistent bowel infection;
- medication side effects.
IBS is especially easy to confuse with lactose intolerance because symptoms commonly include pain, bloating and changes in bowel habit.
NICE recommends considering other diagnoses such as coeliac disease when assessing suspected IBS rather than endlessly testing and restricting foods.
Our broader guide to digestive symptoms and common conditions may help if your symptoms do not fit one clear food trigger.
Seek medical advice if you have persistent symptoms, particularly if you also develop:
- unexplained weight loss;
- blood in your stool;
- ongoing diarrhoea;
- persistent vomiting;
- significant weakness or anaemia;
- severe abdominal pain;
- a major new change in bowel habit.
If diarrhoea itself has become the main problem, see our guide to diarrhoea and vomiting in adults.
Frequently Asked Questions
Can you suddenly become lactose intolerant?
Yes. Lactase production can reduce gradually with age, so symptoms may become noticeable in adulthood. Lactose intolerance can also develop temporarily or permanently after damage to the small intestine from an infection, coeliac disease, inflammatory bowel disease or other conditions.
How quickly do lactose intolerance symptoms start?
Symptoms can begin within minutes or several hours after consuming lactose. Timing depends on how much lactose was eaten, how quickly food moves through the digestive system and the degree of lactase deficiency.
Does lactose intolerance always cause diarrhoea?
No. Some people mainly experience bloating, wind, rumbling and cramps. Others develop loose stools or urgency. The pattern varies between individuals.
Can lactose intolerance cause constipation?
Although diarrhoea is more typical, the NHS includes constipation among possible symptoms in some people. Persistent constipation has many other potential causes, so it should not automatically be blamed on lactose.
Can I eat cheese if I am lactose intolerant?
Often, yes. Mature hard cheeses are naturally very low in lactose and many people tolerate them well even when ordinary milk causes symptoms. Your individual tolerance is what matters.
Can I eat yoghurt?
Possibly. Some people find yoghurt easier to digest than milk because bacterial cultures help break down lactose. Lactose content varies between products, so start with a small amount if you are unsure.
Is lactose-free milk completely dairy-free?
No. Lactose-free cow's milk is still dairy and still contains cow's milk proteins. It is suitable for lactose intolerance but generally not for people who need to avoid milk protein because of allergy.
Is lactose intolerance the same as IBS?
No. Lactose intolerance results from difficulty digesting lactose. IBS is a broader disorder involving abdominal pain and altered bowel habits. The symptoms can overlap and some people can have both.
Can coeliac disease cause lactose intolerance?
Yes. Coeliac disease can damage the lining of the small intestine where lactase is produced. This can cause secondary lactose intolerance. Lactose tolerance may improve as the intestine heals after appropriate treatment of coeliac disease.
Do I need a breath test to diagnose lactose intolerance?
Not necessarily. A clear improvement after reducing lactose followed by recurrence when lactose is reintroduced may provide useful evidence. Breath testing can help when the diagnosis is uncertain or symptoms are more complicated.
Are home food intolerance tests accurate?
Many commercial tests claiming to diagnose multiple food intolerances are not recommended. The NHS says there is limited evidence that these tests accurately identify food intolerance and warns that they can lead people to unnecessarily exclude numerous foods.
Do lactase tablets really work?
They can reduce symptoms for some people by supplying the enzyme needed to break down lactose. Their effectiveness depends on how much lactose you consume and the amount of lactase taken.
Can lactose intolerance go away?
Primary lactose intolerance caused by naturally reduced lactase production usually persists, although symptoms can often be controlled easily. Secondary lactose intolerance caused by bowel infection or another treatable intestinal condition may improve when the bowel recovers.
Do I need to completely stop eating dairy?
Usually not. Many people with lactose intolerance can tolerate smaller portions or low-lactose foods. Completely excluding dairy may make it harder to obtain enough calcium and other nutrients, so dietary restriction should be proportionate to your symptoms.
This guide provides general information and is not a diagnosis. Recurrent digestive symptoms can have many causes. Speak to a GP or dietitian if symptoms are persistent, severe, associated with weight loss or bleeding, or require you to substantially restrict your diet.
