That rotten egg smell comes from hydrogen sulfide, a sulfur-containing gas produced when bacteria in your large intestine break down sulfur-rich foods. Everyone produces some hydrogen sulfide during digestion, but the intensity of the smell depends on what you eat, how much protein reaches your colon undigested, and which bacteria dominate your gut. Most of the time, it’s completely normal and diet-driven.
How Your Gut Produces Hydrogen Sulfide
Your large intestine hosts a community of bacteria that ferment whatever your small intestine didn’t fully absorb. When those bacteria encounter sulfur-containing compounds, they reduce them into hydrogen sulfide, the same gas responsible for the smell of rotten eggs, hot springs, and volcanic vents. Even at tiny concentrations, your nose picks it up immediately.
The bacteria most responsible belong to a group called sulfate-reducing bacteria. The most common species in the human gut uses sulfate as fuel for its metabolism and releases hydrogen sulfide as a waste product. Everyone carries some of these bacteria, but their population size varies from person to person, which partly explains why some people consistently produce smellier gas than others. Your intestinal lining also generates small amounts of hydrogen sulfide on its own, though the bacteria in your colon are the primary source.
Foods That Make It Worse
The biggest dietary driver is sulfur content. Cruciferous vegetables like broccoli, Brussels sprouts, and cabbage are especially high in sulfur compounds. When gut bacteria break these down, hydrogen sulfide production spikes. Other sulfur-rich foods include eggs, garlic, onions, nuts, meat, dairy, beer, and wine. If you had a steak dinner with a side of broccoli and a glass of red wine, your gut bacteria just got a buffet of sulfur substrates.
Protein intake matters independently of sulfur content. A controlled feeding study found that fecal sulfide concentrations increased more than fifteenfold when participants went from a zero-meat diet to a high-meat diet. Dietary protein from meat is one of the most important substrates for sulfide generation in the large intestine. This is because protein that isn’t fully digested in your small intestine travels to the colon, where bacteria ferment it and release sulfur-containing gases. High-protein diets, protein shakes, and meals heavy in red meat can all increase the egg-like smell noticeably.
Why It’s Sometimes Worse Than Usual
A sudden change in smell often traces back to a recent dietary shift. Starting a high-protein diet, eating more cruciferous vegetables, or increasing your beer or wine intake can all tip the balance. Your gut bacteria adapt to your usual diet over time, so a dramatic change gives them a sudden flood of new substrates to ferment.
Constipation also plays a role. When stool moves slowly through the colon, bacteria have more time to ferment its contents, producing more gas in the process. Anything that slows transit, from dehydration to travel to certain medications, can make your gas smell stronger even without a dietary change.
Food intolerances are another common culprit. If you have trouble digesting lactose or certain carbohydrates, more undigested material reaches the colon, feeding bacterial fermentation. The result is often both increased gas volume and a stronger sulfur smell.
When Smelly Gas Signals Something Deeper
Persistently foul-smelling gas can occasionally point to a digestive condition worth investigating. People with ulcerative colitis, for example, release three to four times more hydrogen sulfide in their stool compared to healthy individuals. Their colons harbor higher numbers and greater activity of sulfate-reducing bacteria. This doesn’t mean smelly gas causes colitis, but chronically elevated hydrogen sulfide can irritate the intestinal lining.
For context, passing gas 14 to 23 times a day falls within the normal range. If you’re consistently exceeding that, or if the smell is accompanied by persistent bloating, stomach pain that won’t resolve, ongoing diarrhea or constipation, unexplained weight loss, or blood in your stool, those patterns warrant a conversation with your doctor.
How to Reduce the Smell
The most effective approach is dietary. Cutting back on the highest-sulfur foods, particularly red meat, eggs, cruciferous vegetables, garlic, and onions, for a week or two will usually produce a noticeable difference. You don’t need to eliminate these foods permanently. Try reducing them one at a time to identify which ones affect you most, since individual gut bacteria compositions mean different foods trigger different people.
Bismuth subsalicylate, the active ingredient in Pepto-Bismol, binds more than 95 percent of sulfide gases in the gut. It’s the most effective over-the-counter option specifically for odor rather than gas volume. It won’t reduce how often you pass gas, but it significantly reduces how bad it smells.
Staying hydrated and eating enough fiber helps keep stool moving through the colon at a normal pace, reducing the time bacteria have to produce hydrogen sulfide. Regular physical activity supports gut motility for the same reason. If you’re on a high-protein diet, spacing your protein intake across multiple meals rather than loading it into one or two large servings gives your small intestine a better chance of absorbing it before it reaches the colon, leaving less for bacteria to ferment.