The essentials

Lactose intolerance means milk sugar (lactose) causes bloating, gas, cramps or diarrhea because the gut makes too little lactase. It is very common, not dangerous, and most people can still have some dairy. The best test is a hydrogen breath test; routine blood tests do not show it. Hives or wheezing after milk point to a milk allergy instead.

What is lactose intolerance, at a glance?

  • What it is: digestive symptoms after eating or drinking lactose, the natural sugar in milk[1].
  • Malabsorption is not the same as intolerance: many people digest lactose poorly without any symptoms. Only people who have symptoms are called lactose intolerant[1].
  • How common: about 65 percent of people worldwide have a reduced ability to digest lactose after infancy[8]. It is most frequent in people of East Asian descent and least frequent in people of Northern European descent (about 5 percent)[8].
  • Key tests: the hydrogen breath test first; the lactose tolerance blood test and the LCT genetic test in some cases[5][9].

What are the symptoms of lactose intolerance?

Symptoms often start 30 minutes to 2 hours after milk products[7]. They include:

  • bloating, gas and stomach rumbling;
  • abdominal pain or cramps;
  • loose stools or diarrhea;
  • nausea, and sometimes vomiting[2].

Symptoms can be mild or severe, depending on how much lactose you have[2]. Lactose intolerance does not cause blood in the stool, fever, weight loss or night-time diarrhea: those signs point to another cause and need a medical check.

Feeling bad after dairy does not prove lactose intolerance: about half of people who digest lactose normally report discomfort after an open lactose drink[10]. Testing helps before you cut out dairy for good.

When should I see a doctor?

When to see a doctor

  • Emergency: call 911 if, minutes after milk or dairy, there is swelling of the lips, tongue or throat, trouble breathing, widespread hives or fainting. This is an allergic reaction, not lactose intolerance.
  • Within 24 to 48 hours for diarrhea with fever or blood, diarrhea that keeps a baby or older person from drinking, or severe pain that does not settle.
  • At your next appointment if symptoms get worse or do not improve with less lactose, if you plan to cut out dairy for a long time, or if a child under 2 or 3 has symptoms or is not growing or gaining weight well[7].

What causes lactose intolerance?

Lactose that is not absorbed reaches the large intestine, where bacteria ferment it. This makes gas (hydrogen, sometimes methane) and draws water into the bowel[10].

  • Common Primary lactase deficiency (lactase non-persistence): the most common cause. Lactase levels fall after infancy, so symptoms may only appear in later childhood, the teenage years or early adulthood[2]. It is inherited: a variant in a control region of the MCM6 gene, next to the LCT (lactase) gene, keeps lactase working into adulthood in some people[8].
  • Possible Secondary lactose intolerance: an injury to the small intestine lowers lactase[2]. Causes include infections such as gastroenteritis or intestinal parasites like giardia, celiac disease, Crohn's disease, and some medicines, surgery or radiation[2][12]. Tolerance may come back once the cause is treated[2]. In young children, lactose intolerance is usually of this kind, and primary lactose intolerance rarely shows before age 5[12].
  • Rare Premature birth: a premature baby may make too little lactase for a short time after birth[2].
  • Rare Congenital lactase deficiency: the intestine makes little or no lactase from birth[2].

People with irritable bowel syndrome (IBS) are more likely to have symptoms after lactose, and their symptoms are often stronger, even when they digest lactose normally[10].

How is lactose intolerance diagnosed? Tests and how to read them

Doctors start with your symptoms, family history and eating habits, and may suggest stopping milk products for a while to see whether symptoms go away[3]. IBS, celiac disease, inflammatory bowel disease and bacterial overgrowth can look the same[3].

  • Hydrogen breath test (the preferred test): you breathe into a bag, drink a lactose solution, then give a breath sample about every 30 minutes for a few hours[5]. European guidelines use 25 g of lactose, 3 to 5 hours of testing, and a rise of 20 parts per million (ppm) of hydrogen above the starting value as the sign of malabsorption[9]. A rise in methane of 10 ppm or more may also count[9]. Your symptoms during the test are part of the result: malabsorption plus symptoms means intolerance[9].
  • Lactose tolerance blood test: blood glucose is measured before and usually at 30 minutes, 1 hour and 2 hours after a lactose drink[5], often 50 g[10]. A rise of more than 30 mg/dL within 2 hours is normal, 20 to 30 mg/dL is inconclusive, and less than 20 mg/dL is abnormal[6]. It is less reliable if you have diabetes, because glucose may rise even when lactose is poorly absorbed[5]. It differs from the oral glucose tolerance test.
  • Genetic test (LCT-13910 C/T): a blood or cheek-swab test for the variant linked to lactase persistence. Only a C/C result is considered lactase non-persistence[10]. It does not detect secondary lactose intolerance, and it is not advised for clinical use in people of African or Asian heritage, in whom other variants matter[10].
  • In babies and young children: a stool pH test can show acidic stools from unabsorbed sugar; a small bowel biopsy is rarely needed[7].
Fictitious example of a lactose breath test and genetic test report, adult (United States format)
TestResultReference rangeFlag
Breath hydrogen, baseline (fasting)6 ppm<20 ppm
Breath hydrogen, peak (120 min after 25 g lactose)48 ppmSee rise
Hydrogen rise above baseline42 ppm<20 ppmH
Methane rise above baseline3 ppm<10 ppm
Symptoms during testBloating and cramps from 90 minNoneA
LCT-13910 C>T genotypeC/CC/T or T/TA
  1. Rise of 42 ppm, flag H: more than the 20 ppm limit, so part of the lactose was not absorbed (malabsorption)[9].
  2. Symptoms during the test: malabsorption plus typical symptoms is what defines lactose intolerance[9]. A rise without symptoms means malabsorption only.
  3. C/C genotype: fits primary lactase non-persistence. It confirms the likely cause but not, on its own, that lactose explains your symptoms.
  4. Reference ranges: examples only; the limits and protocol printed on your own report are the ones that count.

Breath test, genetic result or a full lab report after weeks of bloating? BloodSense explains every line in plain words and tells you what to ask your doctor next.

Explain my test results

What a negative test means: lactose is probably not the cause, but it is not fully ruled out[5]: in at least 10% of adults, gut bacteria make little hydrogen[9].

What blood tests do not show: a complete blood count, a metabolic panel or a fasting glucose do not diagnose lactose intolerance. Food "intolerance" panels that measure IgG or IgG4 antibodies to milk are not recommended: these antibodies reflect normal exposure to food, not intolerance[11]. An IgE blood test to milk looks for a milk allergy, a separate immune condition that lactose intolerance is often confused with[12].

What follow-up is needed?

Primary lactose intolerance needs no repeat testing. If the intolerance is secondary, the doctor follows the underlying condition; lactose tolerance may return once it is treated[2]. If you avoid dairy for a long time, the main point to watch is getting enough calcium and vitamin D, because low calcium can weaken bones over time (osteoporosis)[1]. Your doctor may check vitamin D levels if you have risk factors; blood calcium does not reflect what you eat.

How is lactose intolerance treated?

  • Less lactose, not zero: cutting down on milk products most often eases symptoms[7]. Cutting out lactose completely is not required[10]. Many people can have about 12 grams of lactose, the amount in about 1 cup of milk, with no or mild symptoms[4].
  • Dairy products with the lactose removed or reduced: they are just as healthy as regular milk products[4].
  • Lactase products: tablets or drops taken with dairy help digest lactose[4]. Their effect on symptoms is real but modest[10].
  • If you also have IBS: a wider diet that limits several poorly absorbed sugars (low-FODMAP), guided by a dietitian, tends to work better than cutting lactose alone[10].
  • Babies: a suspected cow's milk allergy needs a different, strict diet decided with a doctor[12].

Living with lactose intolerance: what can I eat?

  • Read labels: milk can be added to boxed, canned, frozen and prepared foods; look for whey, curds, milk by-products and dry milk solids[4].
  • Keep calcium up: adults need 1,000 to 1,500 mg of calcium a day depending on age and sex[7]. Non-dairy sources include canned salmon or sardines with bones, leafy greens and tofu[4].
  • Do not forget vitamin D, which helps the body absorb calcium[4].

Inherited lactose intolerance cannot be prevented. If wheat also seems to cause symptoms, see gluten intolerance and its tests: they are completely different.

Questions to ask your doctor

  1. Is a breath test useful in my case, or should I first try a period with less lactose?
  2. Could my symptoms come from IBS, celiac disease or an infection instead?
  3. Would a genetic test add anything, given my family origin?
  4. How much dairy can I keep, and how do I get enough calcium and vitamin D?

Frequently asked questions

Is lactose intolerance the same as a milk allergy?

No. Lactose intolerance is a digestive problem with the sugar in milk. A milk allergy is an immune reaction to milk proteins; it can cause hives, vomiting or breathing trouble and needs strict avoidance[12].

Can you become lactose intolerant as an adult?

Yes. Lactase often declines slowly, so symptoms may appear in the teenage years or adulthood. A bowel infection or another gut disease can also cause it at any age[2].

Is there a cure for lactose intolerance?

Primary lactose intolerance is lifelong, but symptoms are controlled by adjusting how much lactose you have. Secondary intolerance can go away when its cause is treated[2].

Are home lactose intolerance tests reliable?

A breath test kit prepared by a laboratory follows the same principle as a clinic test. Panels measuring IgG antibodies to foods are not reliable for intolerance[11].

  1. National Institute of Diabetes and Digestive and Kidney Diseases. Definition and facts for lactose intolerance. Last reviewed February 2018. niddk.nih.gov (accessed November 3, 2026)
  2. National Institute of Diabetes and Digestive and Kidney Diseases. Symptoms and causes of lactose intolerance. Last reviewed February 2018. niddk.nih.gov (accessed November 3, 2026)
  3. National Institute of Diabetes and Digestive and Kidney Diseases. Diagnosis of lactose intolerance. Last reviewed February 2018. niddk.nih.gov (accessed November 3, 2026)
  4. National Institute of Diabetes and Digestive and Kidney Diseases. Eating, diet, and nutrition for lactose intolerance. Last reviewed February 2018. niddk.nih.gov (accessed November 3, 2026)
  5. MedlinePlus. Lactose tolerance tests. Last updated September 30, 2024. US National Library of Medicine. medlineplus.gov (accessed November 3, 2026)
  6. MedlinePlus Medical Encyclopedia. Lactose tolerance tests. Reviewed August 12, 2024. US National Library of Medicine. medlineplus.gov (accessed November 3, 2026)
  7. MedlinePlus Medical Encyclopedia. Lactose intolerance. Reviewed April 27, 2026. US National Library of Medicine. medlineplus.gov (accessed November 3, 2026)
  8. MedlinePlus Genetics. Lactose intolerance. Last updated March 24, 2023. US National Library of Medicine. medlineplus.gov (accessed November 3, 2026)
  9. Hammer HF, Fox MR, Keller J, et al. European guideline on indications, performance, and clinical impact of hydrogen and methane breath tests in adult and pediatric patients: European Association for Gastroenterology, Endoscopy and Nutrition, European Society of Neurogastroenterology and Motility, and European Society for Paediatric Gastroenterology Hepatology and Nutrition consensus. United European Gastroenterol J. 2022;10(1):15-40. doi:10.1002/ueg2.12133 (accessed November 3, 2026)
  10. Misselwitz B, Butter M, Verbeke K, Fox MR. Update on lactose malabsorption and intolerance: pathogenesis, diagnosis and clinical management. Gut. 2019;68(11):2080-2091. doi:10.1136/gutjnl-2019-318404 (accessed November 3, 2026)
  11. Stapel SO, Asero R, Ballmer-Weber BK, et al. Testing for IgG4 against foods is not recommended as a diagnostic tool: EAACI Task Force Report. Allergy. 2008;63(7):793-796. doi:10.1111/j.1398-9995.2008.01705.x (accessed November 3, 2026)
  12. Heine RG, AlRefaee F, Bachina P, et al. Lactose intolerance and gastrointestinal cow's milk allergy in infants and children: common misconceptions revisited. World Allergy Organ J. 2017;10(1):41. doi:10.1186/s40413-017-0173-0 (accessed November 3, 2026)

This page is general information. It does not replace the advice of a doctor, who interprets your tests with your symptoms and history and decides what to do next.