The smell comes from sulfur. Sulfur compounds make up only about 1% of the gas you pass, but they’re entirely responsible for the odor. The other 99%, mostly nitrogen, hydrogen, carbon dioxide, and methane, is completely odorless. So when your farts are particularly stinky, it means your gut is producing more of that tiny sulfur fraction than usual. Healthy adults pass gas 8 to 14 times a day on average, and up to 25 times is still considered normal, so the real question isn’t how often but why the smell changes.
What Creates the Smell
Your large intestine is home to trillions of bacteria that break down whatever your small intestine couldn’t fully digest. When these bacteria ferment leftover carbohydrates, the byproducts are mostly odorless gases like hydrogen, carbon dioxide, and methane. But when they go to work on sulfur-containing compounds, the result is hydrogen sulfide, the gas that smells like rotten eggs. A specific group called sulfate-reducing bacteria are the main producers, and their activity ramps up whenever more sulfur-rich material reaches the colon.
Other trace sulfur gases contribute too, including methanethiol (which has a decaying-cabbage smell) and dimethyl sulfide. Even in tiny concentrations, these compounds register strongly to the human nose. That’s why a single especially pungent episode can clear a room despite being 99% odorless gas by volume.
Foods That Make It Worse
The biggest dietary driver of smelly gas is protein, particularly animal protein. In a controlled feeding study, fecal sulfide concentrations rose dramatically as meat intake increased, jumping from 0.22 mmol/kg on a meat-free diet to 3.38 mmol/kg when participants ate 600 grams of meat per day. That’s roughly a 15-fold increase. The sulfur-containing amino acids in meat, eggs, and dairy give gut bacteria exactly the raw material they need to churn out hydrogen sulfide.
Cruciferous vegetables like broccoli, cauliflower, cabbage, and Brussels sprouts are another well-known trigger. These contain both sulfur compounds and complex carbohydrates that resist digestion in the small intestine, delivering a double dose of fermentable material to colonic bacteria. Garlic, onions, and legumes follow the same pattern. Foods and drinks preserved with sulfites, including wine, dried fruit, and some processed meats, add inorganic sulfur to the mix.
A week-long plant-based diet produced measurably less hydrogen sulfide than a week-long animal-based diet in most people studied, with a median difference of 29 parts per million per gram. That said, about 18% of participants actually produced more hydrogen sulfide on the plant-based diet, which highlights how much individual gut bacteria composition matters.
Your Gut Bacteria Are Unique
Two people can eat the same meal and produce very different levels of odor. The reason is that everyone’s microbiome, the specific mix of bacterial species in the colon, is different. People with higher populations of sulfate-reducing bacteria will convert more dietary sulfur into hydrogen sulfide. These bacteria use hydrogen (produced by other microbes fermenting carbohydrates) along with sulfate as fuel, and when both are available, they outcompete other bacterial groups for resources.
Interestingly, shifting your diet doesn’t necessarily change the number of sulfate-reducing bacteria in your gut. Instead, it changes how much sulfur substrate they have to work with. So while your bacterial lineup stays relatively stable in the short term, the raw materials you feed those bacteria day to day have a direct effect on how your gas smells.
Slow Digestion Intensifies Odor
When food moves slowly through your colon, bacteria have more time to ferment it. Constipation and sluggish transit mean stool sits longer in the large intestine, which leads to extended fermentation and greater gas production. The longer the bacteria work on sulfur-containing residues, the more hydrogen sulfide accumulates.
This is why people who are constipated often notice their gas smells worse than usual, even without any dietary changes. Restoring regular bowel movements, whether through fiber intake, hydration, or physical activity, reduces that extra fermentation window and typically brings the smell back to baseline.
Conditions That Cause Foul-Smelling Gas
Sometimes especially smelly gas points to an underlying digestive issue rather than just last night’s dinner. When your body can’t properly absorb certain nutrients, undigested food reaches the colon in larger quantities, giving bacteria more to ferment.
Lactose intolerance is one of the most common examples. If you lack the enzyme to break down lactose in the small intestine, dairy products pass into the colon largely intact, producing excess gas that’s often particularly foul. Celiac disease works similarly: damage to the small intestine’s lining impairs absorption of multiple nutrients, sending more material downstream for bacterial fermentation.
Small intestinal bacterial overgrowth (SIBO) is another culprit. Normally, the small intestine has relatively few bacteria compared to the colon. When bacterial populations grow there, they start fermenting food earlier in the digestive process, producing both more gas overall and more sulfur-containing compounds. Symptoms typically include bloating, diarrhea, and noticeably smelly flatulence.
Medications That Change Gas Odor
Several common medications can make your gas smellier as a side effect. Anti-inflammatory painkillers like ibuprofen can irritate the gut lining and alter digestion. Some laxatives change how quickly material moves through the colon or shift the bacterial environment. Antifungal medications and statins (used to lower cholesterol) are also linked to increased or more odorous gas. Antibiotics deserve special mention because they can wipe out competing bacterial populations and allow sulfate-reducing bacteria to temporarily dominate, ramping up hydrogen sulfide output until the microbiome rebalances.
How to Reduce the Smell
The most effective lever you have is diet. Cutting back on red meat, eggs, and high-sulfur vegetables for a few days will typically produce a noticeable difference. You don’t need to eliminate these foods permanently. Just be aware that a steak dinner followed by a broccoli side dish is a recipe for potent gas the next day.
Eating more slowly and chewing thoroughly helps your small intestine absorb nutrients before they reach the colon. Staying hydrated and getting regular physical activity both support faster transit time, which limits how long bacteria have to produce sulfur gases. Fermented foods like yogurt and kimchi can support microbial diversity, potentially reducing the dominance of sulfate-reducing bacteria over time, though results vary by person.
If you’ve noticed a persistent change in how your gas smells, especially paired with symptoms like unintentional weight loss, blood in your stool, ongoing diarrhea, fever, or abdominal pain that keeps getting worse, those are signs that something beyond normal digestion may be going on. New-onset digestive symptoms in adults over 55 also warrant a closer look.