Bad breath: Is it coming from the gut?

Nov. 7, 2022
Many dental patients are concerned about halitosis. Bad breath can arise from intraoral issues, but it can also come from the gut. Here are five digestive issues that can contribute to halitosis, as well as symptoms, diagnoses, and treatments.


Halitosis, also commonly known as “bad breath,” is a concern of many patients seeking help from the dentist, as bad breath is usually associated with oral conditions. Cavities, gum disease, dry mouth, tonsil stones, poor salivary flow, acidic oral environments, and the consumption of odorous foods can all be a cause of bad breath.

The oral care professional will typically tell a patient to brush, floss, rinse, and drink more water to combat this problem. But what if the problem isn’t coming from the patient’s mouth? In that case the dentist or hygienist may tell the patient there’s nothing wrong. In some cases, they may refer the patient to an ear, nose, and throat (ENT) physician or a gastroenterologist.

Although intraoral conditions can contribute to about 60%–70% of the etiology behind bad breath, the gut can play a major role (20%–30%) in the problem as well. Most people who have bad breath from the digestive system describe a rotten egg/fish/meat smell due to the bacteria in the digestive system that break down food products into sulfa gases. Sulfur-digesting bacteria and other microorganisms utilize sulfur compounds within food, producing hydrogen sulfide, dimethyl sulfide, and methyl mercaptan as an end product. This can make the breath smell bad and cause chronic halitosis. Here is a review of the top five digestive issues accounting for halitosis.

No. 1: Helicobacter pylori (Hpylori

H. pylori is a type of bacteria that exists within the normal gut microbiota, but when it’s out of balance, it can cause duodenal and stomach ulcers.

Symptoms, tests, and treatment for H. pylori

No. 2: Gastroesophageal reflux disease (GERD) 

GERD is a common digestive system disorder that can cause bad breath. GERD is caused by the failure of the muscular valve (sphincter) that separates the lower end of the esophagus from the stomach. This is known as the lower esophageal valve (LES). The LES normally opens to permit food and liquids to pass into the stomach. When this valve becomes weak, however, it can allow stomach acid and contents up into the esophagus, causing GERD and, sometimes, bad breath. H. pylori can also cause and/or exacerbate GERD.

Diagnosis, symptoms, and treatment for GERD

No. 3: Small intestinal bacterial overgrowth (SIBO)

Because the small intestine is designed for nutritional absorption and has fewer digestive enzymes than the stomach, bacterial overgrowth can occur in this organ. Eighty percent of people with irritable bowel syndrome (IBS) also have SIBO, which can develop after gastric infections. Patients who have fructose malabsorption, lactose intolerance, or eat a lot of fiber can develop SIBO. Gasses produced in SIBO are the major cause of complaint and may cause bad breath.

Symptoms, tests, and treatment for SIBO

No. 4. Irritable bowel syndrome (IBS), Crohn’s disease, and celiac disease

IBS is a common disorder of the digestive system. Most commonly, patients suffer recurrent abdominal pain and altered bowel habits, such as constipation, diarrhea, or both. Both Crohn’s and celiac disease may limit digestion, providing more undigested food for sulfur-reducing bacteria to break down. This generates more hydrogen sulfide and can cause halitosis.

Symptoms for IBS, Crohn’s, and celiac disease

No. 5: Digestive system infection (giardiasis, or giardia for short)

A specific digestive system infection can cause bad breath. Giardiasis is a diarrheal disease caused by the parasite Giardia lamblia, which can infect humans via food or water.

Symptoms, transmission sources, and treatment for giardiasis

Other gut problems causing breath issues

These gut problems can also cause bad breath:

  • Gall bladder dysfunction, gallstones, or gall bladder removal
  • Constipation 
  • Gastroparesis
  • Pyloric stenosis

Editor’s note: This article originally appeared in Perio-Implant Advisory, a chairside resource for dentists and hygienists that focuses on periodontal- and implant-related issues. Read more articles and subscribe to the newsletter.

Scott Froum, DDS, a graduate of the State University of New York, Stony Brook School of Dental Medicine, is a periodontist in private practice at 1110 2nd Avenue, Suite 305, New York City, New York. He is the editorial director of Perio-Implant Advisory and serves on the editorial advisory board of Dental Economics. Dr. Froum, a diplomate of the American Board of Periodontology, is a volunteer professor in the postgraduate periodontal program at SUNY Stony Brook School of Dental Medicine. Contact him through his website at or (212) 751-8530.