Sewage-smelling farts come from sulfur gases produced by bacteria in your large intestine. The main culprit is hydrogen sulfide, the same compound responsible for the rotten-egg smell of actual sewage. While some sulfur in your gas is completely normal, a sudden increase or persistent sewage-like odor usually points to something specific in your diet, a shift in your gut bacteria, or occasionally an underlying digestive condition.
Why Sulfur Gas Smells So Bad
Most of the gas you pass is odorless. It’s made up of nitrogen, oxygen, carbon dioxide, hydrogen, and sometimes methane. These gases come from swallowed air and basic bacterial fermentation, and they don’t smell like much of anything. The smell comes from a small fraction of the total gas volume: sulfur-containing compounds, primarily hydrogen sulfide.
Certain species of bacteria in your gut, particularly sulfate-reducing bacteria like Desulfovibrio and Desulfobulbus, thrive in the oxygen-free environment of the large intestine. They feed on sulfur compounds from your food and produce hydrogen sulfide as a byproduct. Other gut microbes, including some Bacteroides and Clostridium species, break down sulfur-containing amino acids (the building blocks of protein) to generate even more of this gas. When these bacteria are especially active or abundant, the result is gas that smells distinctly like sewage or rotten eggs.
The Foods Most Likely to Blame
Diet is the single most common reason for a sudden change in how your gas smells. Sulfur-rich foods give your gut bacteria more raw material to work with, and the more sulfur that reaches your colon, the more hydrogen sulfide gets produced.
The biggest dietary sources of sulfur include:
- Animal proteins: Eggs, red meat, turkey, chicken, and fish are high in the amino acids cysteine and methionine. Animal proteins generally contain higher levels of these sulfur amino acids than plant proteins do.
- Cruciferous vegetables: Broccoli, cauliflower, cabbage, kale, arugula, and radishes.
- Allium vegetables: Garlic, onions, leeks, shallots, and scallions.
- Legumes, nuts, and seeds: These are plant-based sources of methionine that also contribute.
- Whole grains: They contain sulfur in the form of thiamin (vitamin B1).
If you recently loaded up on a steak dinner with garlic, broccoli on the side, and eggs for breakfast, you’ve essentially given your sulfate-reducing bacteria a feast. The sewage smell that follows 12 to 24 hours later is predictable.
Too Much Protein Reaching Your Colon
High-protein diets deserve special attention here. When you eat the right amount of protein for your body, most of it gets digested and absorbed in the small intestine. But when you eat more protein than your small intestine can handle, the excess travels down to the colon, where bacteria break it down through a process called putrefaction. This produces not just hydrogen sulfide but also other foul-smelling molecules like indole and skatole, compounds that smell distinctly like feces.
This is why people on high-protein diets, protein shakes, or heavy meat-based eating plans often notice their gas becomes significantly worse. The fix is straightforward: scale back protein intake to a level your body can actually absorb, and balance meals with fiber-rich carbohydrates that promote healthier fermentation patterns in the gut.
Food Intolerances and Malabsorption
When your body can’t properly digest certain foods, those undigested particles travel to the large intestine, where bacteria ferment them aggressively. This creates excess gas that often smells worse than normal.
Lactose intolerance is one of the most common examples. If you lack enough of the enzyme that breaks down lactose (the sugar in dairy), that lactose arrives in your colon intact and becomes fuel for gas-producing bacteria. Celiac disease works similarly: damage to the small intestine’s lining means carbohydrates, fats, and proteins aren’t absorbed properly, leaving more material for colonic bacteria to ferment. Both conditions can produce gas with a noticeably foul, sulfurous odor.
A clue that malabsorption might be involved is if the smell comes with other symptoms like bloating, cramping, diarrhea, or greasy-looking stools. These signs suggest something beyond simple dietary sulfur is going on.
Bacterial Overgrowth in the Small Intestine
In a healthy digestive system, the small intestine has relatively few bacteria compared to the colon. When bacteria overpopulate the small intestine, a condition sometimes called SIBO (small intestinal bacterial overgrowth), they start fermenting food earlier in the digestive process and can produce large amounts of hydrogen sulfide. This hydrogen sulfide-dominant form of overgrowth is specifically associated with sulfurous, sewage-like gas, along with bloating, abdominal discomfort, and irregular bowel habits.
Reducing dietary sulfur intake can help lower hydrogen sulfide production in these cases. Some research has also shown green tea extract can reduce hydrogen sulfide levels in the gut. Treatment for SIBO itself typically involves targeted antibiotics that address the bacterial overgrowth directly.
Gut Infections
Certain parasitic and bacterial infections can cause intensely foul-smelling gas. Giardia, a waterborne parasite, is a classic example. A Giardia infection produces gas alongside diarrhea, stomach cramps, nausea, and characteristically greasy, foul-smelling stools that may float. People typically pick up Giardia from contaminated water sources, whether from traveling, camping, or swimming in untreated water.
If your sewage-smelling gas appeared suddenly, especially after travel or exposure to questionable water, and it comes with diarrhea or cramping, an infection is worth considering. A simple stool test can identify most parasitic and bacterial culprits.
Medications and Supplements
Several common medications can increase gas production or change its character. Iron supplements and multivitamins are frequent offenders, as are fiber supplements like Metamucil and Citrucel, antacids, opioid pain medications, and even over-the-counter diarrhea medicines like Imodium. These don’t all produce sulfur gas specifically, but they can slow digestion, alter your gut bacteria, or change the fermentation environment in ways that make gas smell worse.
If you started a new supplement or medication around the time the smell changed, that’s a strong clue. Iron supplements are particularly notorious for producing dark, foul-smelling digestive byproducts.
When the Smell Signals Something Deeper
Occasional sewage-smelling gas after a sulfur-heavy meal is normal and not a cause for concern. The pattern worth paying attention to is persistent, unusually foul gas that doesn’t respond to dietary changes, or gas accompanied by other digestive symptoms: ongoing diarrhea, constipation, unexplained weight loss, abdominal pain, or blood in your stool.
If you suspect something beyond diet, common diagnostic tools include breath tests (which can identify lactose intolerance or bacterial overgrowth), stool tests (which check for infections or problems with pancreatic enzyme production), and imaging studies to visualize the digestive tract. These are simple, noninvasive starting points that can quickly narrow down the cause.
Practical Steps to Reduce the Smell
For most people, the fix is dietary. Start by tracking what you eat for a week alongside when the worst-smelling gas hits. Given the 12-to-24-hour transit time, yesterday’s meals are usually responsible for today’s gas. Cut back on the most sulfur-dense foods, especially eggs, red meat, garlic, onions, and cruciferous vegetables, and see if the smell improves within a few days.
If you’re on a high-protein diet, try reducing your intake and replacing some animal protein with plant-based sources, which contain lower levels of sulfur amino acids. Staying well hydrated and eating enough fiber from non-cruciferous sources (fruits, oats, sweet potatoes) supports more balanced fermentation in the colon. Green tea may also help reduce hydrogen sulfide levels if sulfur gas is an ongoing problem for you.
If dietary adjustments don’t make a noticeable difference within two to three weeks, that’s a reasonable point to get tested for food intolerances, bacterial overgrowth, or other digestive conditions that might be driving the problem.