Lactose intolerance occurs when a person is unable to properly digest food or drinks containing lactose because they lack the digestive enzyme lactase. This can result in a number of uncomfortable digestive symptoms such as stomach pain, bloating, nausea, gas, diarrhea, and more.
Most people with lactose intolerance experience symptoms anywhere from 30 minutes to 1 to 2 hours after eating or drinking foods with lactose—like milk, ice cream, cheese, yogurt, and other dairy products. While it is possible to exhibit signs of lactose intolerance from birth, many times the condition does not show up until a person is an adolescent or even an adult.
There are also some bowel issues that can lead to lactose intolerance due to the damage they inflict such as Crohn's disease, celiac disease, and ulcerative colitis. It is even possible to temporarily develop lactose intolerance if you have a stomach bug. For instance, Rotavirus and Guardia have been known to increase the likelihood of lactose intolerance.
Keep reading for everything you need to know about the signs and symptoms associated with lactose intolerance.
Some evidence suggests that diarrhea is the most common symptom you might experience with lactose intolerance. Because people with lactose intolerance often do not have the lactase enzyme, the lactose in the small intestine cannot be digested.
Consequently, the lactose in the dairy products they consume is then fermented by the bacteria in their small intestine and colon. This fermentation can then lead to diarrhea—as well as bloating, gas, and abdominal pain.
Lactose intolerance also can cause constipation. Although most people will experience diarrhea, about one-third will experience constipation instead—especially those who do not produce hydrogen during the hydrogen breath test. This constipation is largely due to the methane their body's gut bacteria produces.
Keep in mind that experiencing bowel irregularity like constipation and diarrhea does not automatically mean you are lactose intolerant. There are a number of other medical conditions that can cause bowel irregularity including irritable bowel syndrome, inflammatory bowel disease, infections, and more. It is important to talk to a healthcare provider for an accurate diagnosis.
Stomach Pain and Bloating
Within 30 minutes to an hour or so of eating foods with lactose, it is not uncommon to experience stomach pain and bloating. These uncomfortable feelings are caused by the fermentation of unabsorbed lactose by your gut's bacterial microflora—which not only slows down your gut transit time and but also increases the pressure in your abdomen.
This build up of gases and pressure in your abdomen often results in stomach pain and cramps. Typically, you will experience this pain around your navel and in the lower half of your belly. Some people have described it as feeling like having a bubble in their tummy while some have said they can feel the gases moving through their digestive system.
While most people assume that eliminating dairy products is the best approach to reducing uncomfortable symptoms of lactose intolerance, there is some evidence that symptoms like stomach pain and bloating can be reduced through a number of different approaches.
One approach that shows promise—but needs more research—is prebiotic therapy, which has shown to alleviate symptoms and promote the growth of beneficial microorganisms. Talk to your healthcare provider to determine if using prebiotics to help treat your lactose intolerance is right for you.
Nausea and Vomiting
It is not uncommon to feel extremely nauseous after eating foods containing lactose. You may even feel like you are going to vomit—especially if you are lactose intolerant and consumed a large amount of food with lactose. In fact, one study of children in Indonesia found that nausea was the second most reported symptom of lactose intolerance.
These abdominal symptoms are caused by the excessive bacterial fermentation of lactose alongside your body's production of short-chain fatty acids in the small bowel. How quickly your symptoms appear will greatly depend on how much lactase your body produces. If you are severely deficient in this digestive enzyme, you may experience nausea more quickly than someone whose body produces some lactase.
Keep in mind that nausea and vomiting—particularly after you eat—can be caused by a number of other conditions. Some possibilities include stomach bugs, motion sickness, pregnancy, and health conditions that impact your gut or nervous system. If you are consistently experiencing nausea and vomiting, it is important to speak with a healthcare provider to determine if you are lactose intolerant or have something else going on.
Flatulence or Gas
When your body ferments the lactose in your colon, it produces gases such as hydrogen, methane, and carbon dioxide. This fermentation can lead to increased flatulence or gas. But, the extent to which you experience gas will vary depending on your much lactase you produce and how your body responds to the lactose you consumed.
While the excess gas your body produces may be uncomfortable—or even embarrassing—it is usually not harmful. Aside from reducing the amount of lactose-containing foods you eat, there are also other remedies for treating excess gas such as eating more slowly or taking an over-the-counter gas medication. That said, it is always best to talk to a healthcare provider before taking anything new, including over-the-counter medications.
Other Potential Symptoms
While the majority of symptoms stemming from lactose-intolerance are digestive, you also could potentially experience other symptoms, including neurological symptoms like headaches and an inability to concentrate. For instance, there is some evidence that headaches—or even migraines—can be due to lactose-intolerance. Fatigue, mouth sores, muscle pain, and joint pains also can be experienced by those with lactose-intolerance.
It's also important to note that lactose intolerance is not the same thing as a milk allergy. While both conditions may be caused by ingesting milk or other dairy products, lactose-intolerance is a digestive issue and milk allergy is a immune system response.
That said, if you have a child who may appear to have lactose intolerance, it is important to also have them evaluated for a milk allergy. Older research indicates as many as 20% of children with symptoms of lactose intolerance actually have a milk allergy.
When you have a milk allergy, your body overreacts to the proteins in milk, which triggers an allergic reaction. Depending on the severity of your milk allergy, your response can include rashes, hives, itching, and swelling. In more severe cases you may even experience trouble breathing, wheezing, and loss of consciousness.
While a food allergy like a milk allergy can be potentially life-threatening, lactose intolerance is not life threatening. You may experience uncomfortable—and sometimes even painful—symptoms, but having lactose intolerance does not put your life at risk when you ingest dairy products.
National Library of Medicine. Lactose intolerance.
National Institute of Diabetes and Digestive and Kidney Diseases. Symptoms and causes of lactose intolerance.
American College of Gastroenterology. Lactose intolerance.
Saha M, Parveen I, Shil BC, Saha SK, Banik RK, Majumder M, Salam MU, Islam AN. Lactose intolerance and symptom pattern of lactose intolerance among healthy volunteers. Euroasian J Hepatogastroenterol. 2016 Jan-Jun;6(1):5-7. doi:10.5005/jp-journals-10018-1156
OMIM Online Catalog of Human Genes and Genetic Disorders. Lactose intolerance, adult type.
Leszkowicz J, Plata-Nazar K, Szlagatys-Sidorkiewicz A. Can lactose intolerance be a cause of constipation? A narrative review. Nutrients. 2022;14(9):1785. doi:10.3390/nu14091785
Lomer MCE, Parkes GC, Sanderson JD. Review article: lactose intolerance in clinical practice - myths and realities: REVIEW: LACTOSE INTOLERANCE IN CLINICAL PRACTICE. Alimentary Pharmacology & Therapeutics. 2007;27(2):93-103. doi:10.1111/j.1365-2036.2007.03557.x
Children's Minnesota. Patient & Family Education Materials. I'm lactose intolerant and got a burning pain after drinking milk.
Arnold JW, Simpson JB, Roach J, Bruno-Barcena JM, Azcarate-Peril MA. Prebiotics for lactose intolerance: Variability in galacto-oligosaccharide utilization by intestinal Lactobacillus rhamnosus. Nutrients. 2018;10(10):1517. doi:10.3390/nu10101517
Hegar B, Widodo A. Lactose intolerance in Indonesian children. Asia Pac J Clin Nutr. 2015;24 Suppl 1:S31-40. doi:10.6133/apjcn.2015.24.s1.06
Nutrition Issues in Gastroenterology. Got lactase? A clinician's guide to lactose intolerance.
Singh P, Yoon SS, Kuo B. Nausea: A review of pathophysiology and therapeutics. Therap Adv Gastroenterol. 2016;9(1):98-112. doi:10.1177/1756283X15618131
Deng Y, Misselwitz B, Dai N, Fox M. Lactose intolerance in adults: Biological mechanism and dietary management. Nutrients. 2015;7(9):8020-8035. doi:10.3390/nu7095380
Robles L, Priefer R. Lactose intolerance: What your breath can tell you. Diagnostics (Basel). 2020;10(6):412. Published 2020 Jun 17. doi:10.3390/diagnostics10060412
Food Allergy Research and Education. Milk allergy vs. lactose intolerance.