Sulfur-smelling farts come from tiny amounts of sulfur-containing gases produced when gut bacteria break down certain foods. The rotten-egg smell is almost always hydrogen sulfide, which typically makes up only about 50 parts per million of any given fart, yet it’s potent enough to dominate the experience. In most cases, the cause is dietary and completely normal.
Why Farts Smell Like Sulfur
About 99% of intestinal gas is odorless. Roughly a quarter is swallowed oxygen and nitrogen, and the rest is mostly carbon dioxide, hydrogen, and methane produced by gut bacteria. The smell comes entirely from that remaining fraction: trace sulfur compounds, primarily hydrogen sulfide (the classic rotten-egg gas), along with methanethiol (rotten cabbage) and dimethyl sulfide (garlic-like). These three gases together average just 50 parts per million of each fart, but human noses are extraordinarily sensitive to them.
The gas is produced when specific bacteria in your large intestine feed on sulfur-containing compounds from food. A group called sulfate-reducing bacteria, with Desulfovibrio piger being the most common species in the human gut, specializes in this process. These microbes use sulfate as an energy source and release hydrogen sulfide as a byproduct. Everyone has these bacteria to some degree. How much sulfur gas you produce depends on how much sulfur-rich food reaches your colon and how active your particular bacterial population is.
Foods That Make It Worse
The biggest dietary drivers fall into a few categories. Animal proteins like turkey, beef, eggs, fish, and chicken are rich in methionine and cysteine, two sulfur-containing amino acids. When you eat more protein than your small intestine fully absorbs, the excess reaches your colon, where bacteria convert it into hydrogen sulfide. Eggs are a particularly common culprit because they’re high in both amino acids.
Cruciferous vegetables, including broccoli, cauliflower, cabbage, kale, arugula, and radishes, deliver sulfur in the form of glucosinolates. Allium vegetables like garlic, onions, leeks, scallions, and shallots are packed with sulfides, thiosulfates, and sulfoxides. These are all healthy foods, but they feed the exact bacterial pathways that generate smelly gas.
Other contributors include legumes (chickpeas, lentils), whole grains, nuts, and seeds. Beer and wine contain sulfites that can add to the load. If you ate a meal heavy in any combination of these foods in the last 12 to 24 hours, that’s the most likely explanation for a sulfur spike.
Digestive Conditions Linked to Sulfur Gas
When sulfur-smelling gas is persistent, happens regardless of what you eat, or comes with other symptoms, a digestive condition could be involved.
Irritable bowel syndrome (IBS) is one of the most common explanations for chronic gas complaints. It often causes bloating, abdominal cramping, urgency, diarrhea or constipation, and a feeling of incomplete emptying. IBS can overlap with food intolerances and other conditions, which is why the pattern of symptoms can feel inconsistent from week to week.
Small intestinal bacterial overgrowth (SIBO) happens when bacteria that normally live in the large intestine colonize the small intestine, where they ferment food too early in the digestive process. This can produce significant gas, bloating, and strong odor. SIBO becomes more likely if bloating is your dominant symptom, if it worsens specifically after carbohydrates, or if you have a history of motility problems, constipation-dominant IBS, prior abdominal surgery, or symptoms that come and go in cycles.
Lactose and other food intolerances leave undigested sugars in the colon for bacteria to ferment. If sulfur-smelling gas consistently follows dairy, wheat, or specific fruits, an intolerance is worth investigating.
Research has also found that fecal hydrogen sulfide concentrations and sulfate reduction rates are higher in people with ulcerative colitis than in healthy individuals, with a demonstrated link between inflammatory gut conditions and elevated Desulfovibrio abundance. This doesn’t mean smelly gas equals colitis, but persistent changes in gas odor alongside other symptoms deserve attention.
Medications That Can Cause It
GLP-1 medications used for weight management and type 2 diabetes (the class that includes semaglutide and tirzepatide) are a well-known trigger for sulfur-smelling gas and burps. These drugs work by slowing digestion, which keeps food in the stomach longer. When food sits for an extended period, it releases sulfur-containing gases before moving further through the digestive tract. If you started one of these medications recently and noticed a change, that’s a likely connection.
Sulfur-containing supplements, high-dose fiber supplements, and certain antibiotics that disrupt gut bacterial balance can also shift gas composition toward more hydrogen sulfide.
How to Reduce Sulfur Smell
The most straightforward approach is a temporary reduction in high-sulfur foods. You don’t need to eliminate entire food groups permanently. Try cutting back on eggs, cruciferous vegetables, garlic, onions, and red meat for a week or two and see if the smell improves. This tells you whether diet is the main driver.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol, has been shown to bind more than 95% of sulfide gases in the gut. It’s an over-the-counter option that works specifically on the odor component rather than on gas volume. For gas and bloating triggered by beans, bran, or certain fruits, alpha-galactosidase (sold as Beano) helps break down the fermentable carbohydrates before bacteria can get to them, though it targets volume more than smell.
Passing gas 14 to 23 times a day is normal. Sulfur smell on its own, especially after a big meal of the foods listed above, is not a medical concern. The smell is a sign of active bacterial fermentation, not a sign something is broken.
Signs Something Else Is Going On
Sulfur gas paired with certain other symptoms warrants a conversation with a gastroenterologist. Those red flags include severe or persistent abdominal pain or bloating, blood in your stool, ongoing diarrhea or constipation that represents a change from your baseline, unexplained weight loss, nausea, or vomiting. If you’re passing gas more than 23 times a day alongside any of these, the gas is likely a symptom of a broader digestive issue rather than the problem itself.