Hydrogen Breath Test for Lactose Intolerance


What is the hydrogen breath test?

The hydrogen breath test is used to identify one of two conditions: lactose intolerance or an abnormal growth of bacteria in the intestine.

What is lactose intolerance?

Lactose intolerance is the inability to digest lactose, the sugar found in milk. Lactose intolerance occurs because the person’s body may not have any lactase, an enzyme that is normally produced by the small intestine. Lactase is needed to digest lactose. Symptoms of lactose intolerance include cramping, bloating, gas, or diarrhea when dairy products are eaten and drunk.

What is bacterial overgrowth in the intestine?

Normally, small amounts of bacteria are found in the intestine. If there is a large increase in the amount of bacteria, food and nutrients are not being absorbed properly.

Bacterial overgrowth can result from a slow transit (passage) of food through the bowels, or from certain medications. Symptoms of bacterial overgrowth may include abdominal pain, bloating, gas, and diarrhea.

Test Details

What happens during the hydrogen breath test?

A breath sample will be collected and tested for the presence of hydrogen. (Normally, very little hydrogen is detected in the breath.) To obtain the sample, you will be asked to blow up a balloon-type bag. You will then be given a pleasant-tasting solution to drink that contains either glucose (sugar) or lactose (the milk sugar). Breath samples are collected every 15 to 20 minutes for up to three hours, as the solution is digested, to see if there is any increase in hydrogen in the breath. Increased hydrogen breath levels mean that there are problems with digestion.

How do I prepare for the hydrogen breath test?

The hydrogen breath test cannot be done following a colonoscopy, barium enema, or any tests that require a bowel prep to be given. The bowel prep will cleanse the bowel of some of the bacteria. You must wait four weeks to have a breath test done to allow the bacteria to re-establish.

  • 4 weeks before the test: DO NOT take any antibiotics or Pepto Bismol®. Do NOT undergo colonoscopy, barium enema, or other tests that require a bowel prep or cleansing.
  • 24 hours before the test: DO NOT smoke the day before and the day of the test. Smoking can affect the results of the test. Try to avoid places where people smoke. Second-hand smoke can also affect the results of the test.
  • Day of the test: DO NOT smoke. Avoid second-hand smoke. DO NOT chew gum. DO NOT use mouthwash. Use only a small amount of water when you brush your teeth.
  • 8 hours before the test: DO NOT eat or drink anything (including water) for 8 hours before the test.
  • Upon arrival for the test: A healthcare provider will explain the test in detail and answer any questions you may have. The testing procedure can last for up to 3 hours.

Results and Follow-Up

What happens after a hydrogen breath test?

  • Early results of the test will be discussed with you.
  • Final test results will be sent to your physician.
  • You may resume your normal activities.
  • You may resume your normal diet unless you have other tests that require dietary restrictions.



  • American Gastroenterological Association. Food Intolerance: Lactose Intolerance Testing. Accessed 5/16/2016.
  • National Institute of Diabetes and Digestive and Kidney Diseases. Lactose Intolerance. Accessed 5/16/2016.
  • Sachdev AH, Pimentel M. Gastrointestinal bacterial overgrowth: pathogenesis and clinical significance. Ther Adv Chronic Dis. 2013 Sep; 4(5): 223–231.
  • American Gastroenterological Association. Food Allergies and Intolerances. Accessed 6/25/2020.

Last reviewed by a Cleveland Clinic medical professional on 05/16/2016.

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