Why Do My Farts Smell So Bad?

There are a few possible reasons why your farts might smell bad.

Just in case you need to hear it: Farting is normal. Most people experience flatulence 13-21 times a day. And, believe it or not, most gas is odor-free. But sometimes, your gas is downright funky.

There are reasons for your not-so-pleasant-smelling gas. For example: "Silent-but-deadly ones, the really smelly guys, are due to fermentation by bacteria in your colon," said Patricia Raymond, MD, Assistant Professor of Clinical Internal Medicine at Eastern Virginia Medical School.

However, sometimes a bad odor can signal a more serious health problem requiring a thorough workup by a healthcare provider.

Here's what you need to know about farts and why they might have an odor.

Woman holding her stomach because of pain.

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Why Do We Experience Gas?

Gas is a healthy, normal byproduct of digestion from swallowing air when we eat. It can also come in different forms other than gas or flatulence: Belching (burping) and bloating are also symptoms that there is gas in your system.

Specifically, the gas from farts is usually a combination of a few gases:

  • Nitrogen
  • Oxygen
  • Hydrogen
  • Carbon dioxide
  • Methane

Those gases have to escape the body's digestive system—either because they've been swallowed (nitrogen and oxygen) or produced from digestion (hydrogen, carbon dioxide, and methane). As a result, you release these gases in the form of farts.

Farts may or may not have an odor. But here are five main reasons why they might be smelly.


The large intestine contains bacteria—as well as some fungi and viruses—which together are known as the gut microbiome. Those bacteria are responsible for helping digestion by breaking down carbs. This process can result in gas.

During digestion, gut bacteria produce sulfur-containing compounds like hydrogen sulfide that create a stench in your gas, explained Dr. Raymond.

"Certain people have a certain type of flora inside of them that causes them to produce more gas or smellier gas," added Frederick Gandolfo, MD, a gastroenterologist at Precision Digestive Care in Huntington, New York.

Specific Foods

The foods you eat can influence the population of bacteria that live in your colon, which then affects your farts, explained Dr. Gandolfo. Some foods are hard for the body to digest completely, leading to gas production.

Dairy Foods With Lactose

Many people enjoy eating dairy foods, such as:

  • Milk
  • Ice cream
  • Cheese
  • Yogurt

However, these foods are often blamed for stinking things up, and rightly so. An estimated 68% of people in the world are deficient in the enzyme needed to digest lactose (aka, lactase), the natural sugar found in dairy products.

For folks who are lactose intolerant, lactose passes through the small intestine without being absorbed, traveling downstream to the colon, where trillions of bacteria "have a little party," said Dr. Raymond. The effect is smelly gas.

Raffinose, Sorbitol, and Fructose

You probably know that beans are a rich source of protein, antioxidants, nutrients, and minerals. But they also get a bad rap for making you pass gas.

Beans contain raffinose family oligosaccharides (RFOs). RFO is a group of sugars that are at least partially responsible for bloating and gas after a bean-heavy meal. This is because the digestive tract lacks an important enzyme needed to break down and digest these sugars.

Other foods that contain raffinose beyond beans include:

  • Broccoli
  • Brussel sprouts
  • Cabbage

Aside from raffinose, other sugars, including sorbitol and fructose, can also be causes of foul flatulence. People who include these sugars as part of their diet can sometimes experience increased gas symptoms.

What Are Some Foods that Might Contain Sorbitol or Fructose?

Sorbitol can be found in sugar-free candy and artificial sweeteners. Fructose is found in:

  • Soft drinks
  • Onions
  • Wheat
  • Artichokes

However, fruit can contain both of these sugars.


Gas or gas symptoms can be the result of constipation. Constipation is essentially when a person has a hard time pooping. One reason that might happen is that stool may be taking longer than normal to pass through the colon.

When you're constipated, feces "hangs around your colon for too long, and then bacteria can really go to town and cause lots of gas," Dr. Gandolfo said. And that's when things can get smelly.

Dr. Gandolfo said that to keep from getting backed up, drink plenty of water, exercise regularly, and include high-fiber foods in your diet. Some foods that include dietary fiber include:

  • Fruits
  • Unsalted nuts and seeds
  • Vegetables
  • Whole grain foods


Certain prescription drugs, vitamins, and supplements can affect your gut. The reasons they cause flatulence differ by drug, and how often they'll make you pass gas (and how badly that gas stinks) varies from one person to the next, explained Oralia Bazaldua, PharmD, Professor of Family and Community Medicine at UT Health San Antonio.

"Each person, along with their provider, will have to weigh the risks versus benefits of the offending agent," said Dr. Bazaldua.

Infections and Disease

Smelly gas alone is usually nothing to worry about, Dr. Gandolfo said. But certain conditions can cause a change or increase in gas.

If you experience a change in your bowel habits or abdominal pain that doesn't go away after passing gas or having a bowel movement, that could be a sign of another condition.

What Conditions May Cause Smelly Farts?

It's important to rule out or treat other conditions that can cause smelly farts, Dr. Gandolfo said, including:

  • A bowel obstruction
  • Inflammatory bowel disease (such as ulcerative colitis and Crohn's disease)
  • Colon cancer (Colorectal cancer)
  • Celiac disease (an immune reaction to gluten)
  • C. difficile (a bacterial infection that causes diarrhea)

How To Get Relief from Gas and Reduce When Farts Smell

Diet and Eating Habit Changes

If specific foods are the culprits for smelly gas, try to avoid the ones that give you trouble. Additionally, keeping a food diary can help determine if a particular food is causing your stinky gas.

Dr. Raymond recalled a patient who had bad gas but only on weekdays. A big bowl of sugar-free candies at their office was the culprit: They would grab some every morning, "and by late afternoon, in a business meeting, she would be passing lots of gas," said Dr. Raymond.

Furthermore, finding ways to swallow less air during and between meals (e.g., avoiding drinking beverages with a straw or sitting down to eat) as well as eating smaller meals more often may be able to help.

Over The Counter Medications

There are a few OTC medications that may be able to offer some relief from gas, including Beano (which helps break down some complex carbs) and bismuth subsalicylate (e.g., Pepto-Bismol), which can help reduce smelly gas.

However, before using any of these medications, it's good to check with a healthcare provider to ensure that the medicines are safe to take considering your health or other medications you may be prescribed.

A Quick Review

Between lactose intolerance, constipation, sugars, infection, and more, there are quite a few reasons your gas may be smelly. If you notice any abnormal changes in your bowels, consult a healthcare provider for more information.

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