Why does my breath smell like rotten eggs?

The Merck Manual explains that most bad breath comes from anaerobic bacteria breaking down food particles, dead cells and proteins in the mouth, producing volatile sulfur compounds such as hydrogen sulfide and methyl mercaptan. Hydrogen sulfide is the rotten-egg gas. Methyl mercaptan smells more like rotten cabbage. Together they explain why people describe their breath as eggs, sulfur, or even like a fart: intestinal gas contains some of the same sulfur compounds, though in your breath they almost always come from the mouth, not the gut.

Most of these bacteria live on the rough back third of the tongue, which a toothbrush rarely reaches, and in pockets around inflamed gums.

1. Tongue coating. The MSD Manual lists odor-causing bacteria on the tongue among the most common causes of bad breath, even in people with healthy gums.
2. Gum disease. Bleeding, swollen gums create pockets where sulfur-producing bacteria thrive. The NHS lists bad breath and a bad taste as results of gum disease.
3. Dry mouth and morning breath. Saliva flow drops overnight and with dehydration, mouth breathing and many medicines.
4. Tonsil stones and postnasal drip. Both trap debris and mucus at the back of the throat.
5. Sulfur burps and reflux. Acid reflux can cause bad breath and a sour taste, according to the NHS, and sulfur-smelling burps come from gas in the stomach.
6. H. pylori stomach infection. A 2016 meta-analysis found bad breath was more common in people with H. pylori and that successful treatment of the infection was linked to improvement.
7. Liver disease. Cleveland Clinic notes methyl mercaptan contributes to fetor hepaticus, whose overall smell is musty and sweet. This is rare and comes with other symptoms.

A simple check: scrape the back of your tongue and smell the scraper after a few seconds. If it smells like your breath, the tongue is a major source.

• Clean the back of your tongue once a day. The NHS recommends a tongue scraper or cleaner. See our step-by-step tongue scraper guide.
• Brush twice a day and clean between teeth daily to deal with gum pockets.
• Drink water, and chew sugar-free gum to keep saliva flowing.
• See a dentist for a cleaning if gums bleed, since tartar below the gum line cannot be removed at home.

On the evidence: a 2019 Cochrane review found only two small trials (46 people) comparing tongue cleaning with no tongue cleaning, and rated that evidence very low certainty, as it did for most other bad-breath treatments. So we cannot say how big the benefit is. Cleaning is cheap and low risk, it targets the most common source, and the NHS includes it in its self-help advice, which is why we still think it is worth doing.

A fecal smell is a related but different complaint, with its own short list of urgent causes. We cover it separately in why your breath smells like poop.