Lactose Intolerance vs. Dairy Allergy

Medically Reviewed by Minesh Khatri, MD on September 11, 2023
4 min read

Lactose intolerance and dairy allergy sound a lot alike. Many people think they’re the same thing. But, how they’re caused (and how they affect your body) are very different.

Lactose intolerance involves the digestive system: If you have it, your body doesn’t make lactase, the enzyme needed to digest lactose. That’s the sugar in milk. Instead of digesting normally in your stomach and small intestine, undigested lactose moves into your colon, where it’s broken down by bacteria and causes bloating and gas. It can be uncomfortable, but it’s not dangerous.

Lactose intolerance is common in adults – about 30 million Americans have it by age 20. It’s more common in people with Asian, African or Native American heritage and less common in people with a northern or western European background.

Dairy allergy involves the immune system: If you have it, your body reacts to the proteins in milk and other dairy products as if they’re dangerous invaders. It releases substances that cause allergy symptoms. This allergic reaction can be mild (rashes) to severe (trouble breathing, loss of consciousness).

Dairy allergy is one of most common allergies, especially in children. As many as 2 in every 100 children under 4 years old are allergic to milk. It’s even more common in babies.

Some symptoms of lactose intolerance and dairy allergy may be the same:

But dairy allergy can also cause a reaction in other parts of your body, including the skin and lungs:

  • Rash
  • Hives
  • Swelling, often in the lips and face
  • Wheezing
  • Tightness in throat
  • Trouble swallowing

There may be blood in the stool (poop) too, especially in babies.

Anaphylaxis is a serious, life-threatening allergic reaction and often begins minutes after you eat a food you’re allergic to, but sometimes it can happen hours later. It usually involves more than one symptom in more than one part of your body at the same time.

If you have a severe allergy or if you’ve ever had anaphylaxis in the past, talk to your doctor about carrying an injectable epinephrine (AdrenaclickAuvi-QEpiPen, a generic auto-injector, Symjepi) to slow down or stop the allergic reaction.

You’re more likely to develop a dairy allergy if:

  • You have other allergies
  • You have eczema
  • One or both of your parents has a food or other allergy, like hay fever, eczema or asthma
  • You’re young. Milk allergy is more common in children. As you get older, your digestive system is less likely to react to milk, but you're more likely to have lactose intolerance.

First your doctor will take your medical history to understand your symptoms and how your body reacts to dairy foods. Then you’ll be tested to confirm whether it’s lactose intolerance or a dairy allergy.

Most adults with probable lactose intolerance are never formally tested.

Lactose tolerance test: You’ll drink a liquid that contains a lot of lactose. About 2 hours later, the amount of glucose (sugar) in your bloodstream will be measured. If your glucose level doesn’t rise, you’re not digesting the lactose in the drink.

Hydrogen breath test: You’ll drink a liquid that contains a lot of lactose. Then, the hydrogen in your breath will be measured at regular intervals. If you’re not digesting lactose, it will be broken down in your colon, releasing hydrogen that can be detected in your breath.

Stool acidity test: Babies and children who can’t be tested otherwise can have their stool tested for lactic acid caused by the breakdown of undigested lactose in the colon.

Skin prick test: A small drop of liquid containing the dairy allergen is placed under your skin on your forearm or back. If a raised bump surrounded by itchy red skin appears, a dairy allergy is likely.

Your doctor might have you take a blood test too, which measures the amount of certain antibodies in your blood.

Both tests can have “false positives.” You can test positive for an allergy even though you really don’t have it. Your allergist will explain the results.

If an allergy is still suspected but not confirmed, your doctor may have you take an oral challenge. You’ll be fed different foods that may or may not contain milk in increasing amounts to see if you react to food that contains milk.

Lactose intolerance is easily managed, mostly by limiting the amount of dairy food and drink you consume. You can also try lactose-reduced ice cream and milk, or take lactase enzyme supplements when you eat dairy products to help your body digest lactose.

If you have a dairy allergy, you’ll need to avoid all dairy foods and other foods that contain dairy products.

Staying safe means reading food labels to see if milk or ingredients containing milk are included. Milk proteins are found in many foods you wouldn’t expect. Some canned tuna, energy drinks, and even chewing gum contain them. And don’t eat lactose-reduced foods if you have a dairy allergy. They still contain the milk proteins that can cause allergic reactions.