Foul-smelling gas comes down to sulfur. Out of the hundreds of compounds in a typical fart, just three sulfur-based chemicals are responsible for most of the stink: one smells like rotten eggs, another like rotten cabbage, and the third like garlic. Together, these make up only about 50 parts per million of each fart, but they’re so potent that even at trace levels they dominate what you smell. The rest of your gas, mostly nitrogen, hydrogen, carbon dioxide, and sometimes methane, is completely odorless.
Where the Sulfur Comes From
Your large intestine is home to trillions of bacteria that ferment whatever your small intestine didn’t fully absorb. A specific group of these microbes, called sulfate-reducing bacteria, consume hydrogen and sulfur-containing compounds left over from your food and produce hydrogen sulfide as a byproduct. The most common of these bacteria in the human gut belongs to the genus Desulfovibrio. The more sulfur-rich material that reaches your colon, the more raw material these bacteria have to work with, and the worse things smell.
Two other compounds also contribute: skatole and indole, which are linked to the smell of feces. Even in tiny amounts, these molecules punch well above their weight in the odor department.
Foods That Make It Worse
If your gas has taken a turn for the worse, your diet is the most likely explanation. Sulfur enters your gut through two main routes: sulfur-containing amino acids in protein (cysteine and methionine, found in all animal proteins) and sulfate compounds in certain plants and drinks.
The biggest dietary contributors include:
- Cruciferous vegetables: broccoli, cauliflower, Brussels sprouts, cabbage, kale, and bok choy
- Allium vegetables: garlic, onions, leeks, shallots, and chives
- Red meat, eggs, and dairy: red meat is the highest in sulfur-containing amino acids, with eggs (both yolk and white) close behind
- Dried fruits: raisins, dates, prunes, and dried apricots
- Beer, wine, and cider: these contain sulfites and sulfate that gut bacteria readily convert to hydrogen sulfide
- Whey protein powder and bone broth: concentrated sources of sulfur-rich amino acids
You don’t need to cut all of these out. But if you recently added more broccoli to your meals, started a whey protein supplement, or have been eating more eggs, that alone could explain a noticeable change in odor.
Food Intolerances and Undigested Sugars
When your body can’t break down a particular sugar in the small intestine, it arrives in the colon intact and becomes a feast for bacteria. The fermentation that follows produces extra gas and can shift the balance of bacterial activity toward more odor-producing species.
Lactose intolerance is the most common version of this. Many people, particularly those of African, Native American, or Asian descent, produce low levels of the enzyme needed to digest lactose. Enzyme levels also decline naturally with age, which is why dairy that never bothered you at 20 might cause problems at 40. Fructose intolerance works similarly: if you can’t fully absorb fructose, fruit, honey, and high-fructose corn syrup will generate more gas than normal.
Other carbohydrates that commonly ferment in the colon include raffinose (found in beans and lentils), sorbitol (a sugar alcohol in diet foods and some fruits), and most starches, including potatoes, corn, and wheat. Dietary fiber is by definition indigestible in the small intestine, so it always reaches the colon and feeds bacteria there. That’s why suddenly increasing your fiber intake often leads to a temporary spike in both gas volume and odor.
Gut Bacteria Imbalances
Your personal mix of gut bacteria determines a lot about how your gas smells. Some people’s microbiomes contain more sulfate-reducing bacteria than others, which means they’ll produce more hydrogen sulfide from the same meal. This bacterial profile isn’t fixed. It shifts with diet, illness, antibiotic use, and time.
Small intestinal bacterial overgrowth (SIBO) is a condition where bacteria that normally live in your large intestine colonize the small intestine instead. Because food meets these bacteria earlier in digestion, it ferments more than it should, producing excess gas along with symptoms like diarrhea, bloating, and sometimes weight loss.
Medications and Supplements
Several common medications increase gas production as a side effect. Fiber supplements like Metamucil and Citrucel, iron pills, multivitamins, opioid pain medications, antacids, and even aspirin can all contribute. Antibiotics deserve special mention because they don’t just increase gas temporarily. They can reshape your gut bacteria for weeks or months, sometimes favoring the sulfur-producing species that make gas smell worse.
Certain supplements popular for joint health, like glucosamine sulfate and chondroitin sulfate, are themselves sulfur compounds. If you started one of these and noticed a change, the timing probably isn’t a coincidence.
Medical Conditions Worth Knowing About
Most of the time, smelly gas is a food issue or a bacterial quirk, not a medical problem. Passing gas 14 to 23 times a day is considered normal, even if some of those are particularly fragrant. But persistently foul gas combined with other symptoms can point to something that needs attention.
Celiac disease causes damage to the small intestine when you eat gluten, leading to malabsorption that sends extra nutrients to colon bacteria. Gastroparesis slows stomach emptying, giving food more time to ferment. Constipation keeps stool sitting in the colon longer, which means more bacterial activity and more sulfur gas building up before it’s released.
Symptoms that suggest something beyond normal variation include unintentional weight loss, blood in your stool, persistent diarrhea or constipation, severe abdominal pain, and heartburn that accompanies the gas.
How to Reduce the Smell
Start with your diet. Keep a simple food log for a week or two, noting what you eat and when your gas is at its worst. Most people can identify a pattern quickly. You don’t need to eliminate sulfur-rich foods entirely, just cut back on the biggest offenders and see if things improve. If you recently started a new protein supplement, fiber product, or joint supplement, try pausing it as a test.
If you suspect lactose intolerance, try removing dairy for a few days. A clear improvement is a strong signal. The same approach works for fructose: cut back on fruit juice, honey, and processed foods with high-fructose corn syrup and track what happens.
Bismuth subsalicylate (the active ingredient in Pepto-Bismol) can reduce gas odor directly. It works by binding to sulfur compounds in your digestive tract, forming a harmless black compound called bismuth sulfide. This is also why it can temporarily turn your tongue and stool black, which is a normal side effect, not a cause for alarm.
Introducing dietary changes gradually rather than all at once also helps. If you’re adding more fiber or vegetables to your diet, ramp up slowly over a couple of weeks. This gives your gut bacteria time to adjust rather than overwhelming them with a sudden flood of fermentable material.