Can You Become Lactose Intolerant?
You enjoy a waffle cone of cookies-and-cream ice cream, and within a few hours you’re experiencing stomach pains, bloating, or diarrhea. You recover, but the same thing happens the next time you have a big serving of dairy.
If that sounds familiar, you could be developing lactose intolerance—even after a lifetime of happily drinking lattes and milkshakes with no problem. It’s a common, natural change in how your body handles dairy, not something you did wrong, and for most people it’s easy to manage.
What causes lactose intolerance?
Lactose is the sugar found in milk. Normally, an enzyme called lactase—which is made in your small intestine—breaks it down so your body can absorb it. Nearly all babies produce plenty of lactase, but most people make less of the enzyme as they get older. Only some of them go on to develop symptoms.
“Because lactose is not broken down by lactase, the sugar that was supposed to be absorbed into your bloodstream instead sits in your small bowel and makes its way through, pulling in water in the gut, which leads to bloating and diarrhea,” explains Hamita Sachar, MD, a Yale Medicine gastroenterologist.
Many people develop lactose intolerance in their 20s and 30s, Dr. Sachar says. “Young people show up in our office and say, ‘I used to drink so much milk and now I can hardly have any without running to the bathroom,’” she says. It can start later in life too, though that’s less common.
Genetics play a role as well. Lactose intolerance is most common in people of East Asian, Native American, and West African descent, and least common in people of Northern European descent—a pattern that traces back to which populations historically raised dairy animals.
Lactose intolerance isn’t the same as a milk allergy, sometimes called a dairy allergy. A milk allergy is an immune reaction to proteins in milk, not the sugar, and it usually shows up in infancy with symptoms like hives, vomiting, trouble breathing, eczema, abdominal pain, or diarrhea. Developing a new milk allergy as an adult is uncommon. Lactose intolerance doesn’t involve the immune system at all—one reason it’s uncomfortable rather than dangerous.
Fortunately, Dr. Sachar says, it’s one of the more manageable conditions she sees. “It’s not harmful,” she adds. “Most people don’t have to fully eliminate dairy from their diets to manage this condition.”
What does lactose intolerance feel like?
The symptoms are gastrointestinal. “It’s usually bloating, cramps, flatulence, and sometimes diarrhea,” says Dr. Sachar, an associate professor of medicine (digestive diseases) at Yale School of Medicine.
Diarrhea means different things to different people, though. “Some people may say multiple bowel movements, others may say it’s very loose,” she says. What matters more is when the symptoms show up and how they compare to your usual pattern—typically within a few hours of eating dairy.
“A change from baseline, that’s the real key—something that’s not your normal pattern,” Dr. Sachar says.
People also have different thresholds for lactose. For some, one glass of milk causes symptoms. For others, it might take three, Dr. Sachar says. The intensity of symptoms can vary depending on how much dairy you consume, sensitivity to other foods consumed at the same time as lactose, and if you have another gastrointestinal condition, such as irritable bowel syndrome.
Do you need to get tested for lactose intolerance?
There is a specific hydrogen breath test that can diagnose lactose intolerance, but Dr. Sachar says she usually doesn’t need it. In most cases, the diagnosis is clear from talking with a patient: their symptoms, their age, and whether lactose intolerance is common in their family or ancestry. She typically reserves the test for situations where the symptoms don’t fit the usual pattern or something more complicated may be going on.
“If the patient is telling me a very clear story of what they are consuming and their symptoms, I have to believe that it’s true,” she says.
The test takes about four hours. It involves drinking a small amount of lactose, then measuring the hydrogen in your breath every hour. Intestinal bacteria produce hydrogen when they break down lactose that hasn’t been absorbed, so a significant rise in breath hydrogen after the lactose drink points to lactose intolerance.
Dr. Sachar typically starts with the simplest approach: cut out dairy and see what happens. If symptoms clear up, that’s the answer. If they don’t improve at all, something else could be going on, such as small intestinal bacterial overgrowth (SIBO) or celiac disease—worth following up on with a doctor.
If eliminating dairy helps, the next step is usually to slowly add dairy back into your diet. “Milk and ice cream have the highest concentration of lactose per serving, so avoiding these or eating an alternative such as lactose-free milk or ice cream, or even nut-based products, is an option,” Dr. Sachar adds.
You can also take a lactase pill right before eating. The pills can be helpful but may not be able to digest all the lactose, Dr. Sachar says, and you can adjust the dose to the size of the serving.
When should you see a doctor?
Lactose intolerance is usually straightforward and not a cause for worry. But it’s worth checking in with your doctor if you also notice:
- Blood in your stool
- Unexplained weight loss
- Fever
- Symptoms that wake you up at night
- No improvement at all after a couple of weeks without dairy
These can point to something else that’s worth ruling out, like SIBO, celiac disease, or inflammatory bowel disease.
Do you have to give up dairy?
Being lactose intolerant doesn’t mean giving up your favorite foods and drinks for good. Beyond lactase pills, lactose content varies a lot from one dairy product to another. A cup of milk has about 12.6 grams of lactose, while a half-cup of cottage cheese has about 3 grams.
The difference comes down to how the food is made. Cheesemaking drains off much of the lactose with the whey, and aging lets bacteria consume more of what’s left—so hard, aged cheeses have less lactose. Fermentation reduces lactose significantly in yogurt, too.
There are also substitutes. “The good thing about 2026 is that there are many alternatives to milk products now—if people want to drink milk, they can drink almond milk, oat milk, or other milk alternatives,” Dr. Sachar says.
Lactose-free milk, which has the lactase enzyme added to it, is another option. The main thing to watch is nutrition: dairy products, including fortified milks, are an important source of vitamin D and calcium. Fortified plant milks are typically enriched with both nutrients, but it’s worth checking the label.
Ultimately, Dr. Sachar says, lactose intolerance is an inconvenience more than anything else. “You do want to work with a doctor and maybe a nutritionist if needed, but this is a benign condition,” she says, recounting words from a doctor she trained under: “‘It’s annoying as hell, but it doesn’t take a day out of your life.’”
If you think you might be developing lactose intolerance, a good first step is talking with your primary care doctor or a gastroenterologist—especially if you want help ruling out other causes or working out a dairy plan that still fits your life.
