The smell comes from sulfur. About 99% of the gas you pass is completely odorless, made up of swallowed air plus carbon dioxide, hydrogen, and methane produced by gut bacteria. The terrible smell is caused by a tiny fraction of sulfur-containing compounds, averaging just 50 parts per million of each fart, that are potent enough to dominate the odor even at trace levels. What you eat, which bacteria live in your gut, and certain medications or health conditions all influence how much of that sulfur gets produced.
The Sulfur Compounds Behind the Smell
Three gases do most of the damage. Hydrogen sulfide produces the classic rotten-egg smell. Methanethiol adds a rotting-cabbage quality. Dimethyl sulfide contributes a garlic-like odor. Together they make up a vanishingly small portion of total gas volume, but your nose can detect them at extraordinarily low concentrations, which is why even a slight uptick in sulfur production makes a noticeable difference.
Two other compounds, indole and skatole, also contribute. These are byproducts of protein breakdown in your colon, and they carry a sharp, fecal smell. Like the sulfur gases, they’re effective at low concentrations. The ratio of all these compounds shifts depending on what you’ve eaten and which bacteria are most active in your gut at any given time, which is why your gas smells different from day to day.
Foods That Make It Worse
Sulfur-rich foods are the most direct trigger. Cruciferous vegetables like broccoli, cauliflower, cabbage, and Brussels sprouts are packed with sulfur-containing compounds called glucosinolates. When you cook these vegetables (which most people do), the plant’s own enzymes for breaking down glucosinolates get deactivated. That means the intact compounds travel all the way to your colon, where bacteria break them apart and release sulfur as a byproduct. The more cruciferous vegetables you eat, the more raw material your gut bacteria have to work with.
High-protein foods, especially eggs and red meat, are another major contributor. These foods contain sulfur-containing amino acids. When more protein reaches your colon than your small intestine could absorb, bacteria ferment it and produce hydrogen sulfide along with indole and skatole. This is why people on high-protein diets often notice their gas smells significantly worse. Garlic, onions, and dried fruits are also high in sulfur compounds and can shift the balance.
Beans and high-fiber foods tend to increase the volume of gas you produce but don’t necessarily make it smell worse. The distinction matters: volume comes from hydrogen, nitrogen, and methane (all odorless), while smell comes specifically from sulfur gases. You might pass gas more often after eating beans, but the individual farts may not smell any worse than usual.
Your Gut Bacteria Matter
Not everyone’s gut produces the same amount of sulfur gas from the same meal. The difference comes down to which bacterial species dominate your colon. A group of bacteria called sulfate-reducing bacteria are the primary producers of hydrogen sulfide. The most common of these belong to a genus called Desulfovibrio, which accounts for roughly 64 to 81% of sulfate-reducing bacteria in the colon. These bacteria consume hydrogen and sulfate and produce hydrogen sulfide as waste.
Other bacteria contribute through different pathways. Some break down the amino acid cysteine directly into hydrogen sulfide. The overall composition of your microbiome, shaped by your long-term diet, antibiotic history, and other factors, determines how much sulfur gas gets generated from any given meal. Two people eating the same dinner can have very different results.
Food Intolerances and Digestive Conditions
Lactose intolerance is a common culprit for increased gas, though it primarily increases volume rather than smell. When you can’t break down lactose in the small intestine, it passes to the colon where bacteria ferment it into carbon dioxide, hydrogen, and methane. The bloating and frequent gas can be dramatic, but the odor increase depends on whether the fermentation also ramps up sulfur-producing bacteria.
A condition called small intestinal bacterial overgrowth, or SIBO, can cause persistently foul-smelling gas. SIBO comes in different forms depending on which gases the overgrown bacteria produce. The hydrogen sulfide type specifically creates sulfur-smelling gas and can also cause a mix of diarrhea and constipation, bloating, and even brain fog. If your gas has been consistently terrible for weeks or months regardless of what you eat, SIBO is worth investigating.
Other conditions that impair digestion, including celiac disease, Crohn’s disease, and chronic pancreatitis, can allow more undigested food to reach the colon, feeding the bacteria that produce smelly gas.
Medications That Change Gas Odor
If you recently started a new medication and noticed your gas (or burps) suddenly smell like sulfur, the drug may be slowing your digestion. GLP-1 medications used for diabetes and weight loss, including semaglutide and tirzepatide, delay stomach emptying. Food sits longer in the digestive tract, giving bacteria more time to ferment it and produce hydrogen sulfide. Sulfur burps affect roughly 3 to 7% of people on these medications depending on the specific drug and dose, and the same mechanism can affect gas further down the tract.
Antibiotics can also temporarily shift your gut bacteria in ways that increase or decrease sulfur production. Iron supplements are another known trigger for changes in gas odor and stool smell.
How to Reduce the Smell
Since smell and volume are caused by different gases, they require different approaches. Most over-the-counter gas remedies target volume, not odor. Simethicone (Gas-X) helps with bloating by breaking up gas bubbles but does nothing for the sulfur compounds that cause smell. Alpha-galactosidase (Beano) helps your body break down the complex carbohydrates in beans and bran before they reach the colon, reducing how much gas you produce, but again, this primarily addresses volume.
For odor specifically, bismuth compounds are one of the few things with evidence behind them. Bismuth reacts with hydrogen sulfide in the colon to form bismuth sulfide, a solid compound that can’t become airborne (it’s what turns your stool black). In clinical studies, patients taking bismuth reported significant improvements in both gas and stool odor. Over-the-counter bismuth subsalicylate (Pepto-Bismol) works through this same mechanism, though it’s not intended for daily long-term use.
Dietary adjustments are the most sustainable fix. Cutting back on cruciferous vegetables, eggs, red meat, garlic, and onions for a week or two can help you identify your personal triggers. You don’t necessarily need to eliminate these foods permanently. Eating smaller portions, cooking vegetables thoroughly, and spreading protein intake across meals rather than loading it into one sitting can all reduce how much sulfur your gut bacteria produce at once.
Probiotics have shown meaningful results for reducing gas volume and bloating. In one trial, a multi-strain probiotic achieved a 50% reduction in flatulence at four weeks compared to placebo. However, most probiotic research has focused on gas volume and frequency rather than odor specifically, so the benefit for smell is less clear.
When Smelly Gas Signals Something Bigger
Foul-smelling gas by itself is almost always a dietary issue, not a medical emergency. But if persistently terrible-smelling gas shows up alongside other symptoms, it can point to a condition that needs attention. The combination of smelly gas with abdominal pain, ongoing diarrhea, unexplained weight loss, blood in your stool, or fever warrants a conversation with a healthcare provider. These patterns can indicate malabsorption disorders, infections, or inflammatory bowel conditions where food isn’t being properly digested before reaching the colon.