Why Do My Farts Smell So Bad?

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

There are reasons for your not-so-pleasant smelling gas: "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.

Gas is a healthy, normal byproduct of digestion from swallowing air when we eat, according to Nemours Children's Health. While the smell may be unpleasant, you are just releasing the swallowed air in the form of gas. However, sometimes a bad odor can signal a more serious health problem requiring a thorough workup by a healthcare provider.

Here are seven reasons why the gas you pass can be smelly.

01 of 07

The Bacteria in Your Gut

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

The foods you eat can influence the population of bacteria that live in your colon, which then affects your farts, explained Frederick Gandolfo, MD, a gastroenterologist at Precision Digestive Care in Huntington, New York.

"Certain people have a certain type of flora inside of them that causes them to produce more gas or smellier gas," Dr. Gandolfo said.

02 of 07

Lactose Intolerance

Dairy foods—like milk, ice cream, and cheese—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, according to the National Institute of Diabetes and Digestive and Kidney Disease.

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

03 of 07

Raffinose

Beans, beans. They're a rich source of protein, antioxidants, nutrients, and minerals, as noted in a 2014 article in The American Journal of Clinical Nutrition. But they 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, according to a February 2022 review in Frontiers in Plant Science.

Whole grains, asparagus, and cruciferous veggies also contain raffinose, according to the International Foundation for Gastrointestinal Disorders.

Taking an enzyme-based digestive aid (like Beano, for example) can help ease symptoms, according to the February 2022 Frontiers in Plant Science article.

04 of 07

Sugars

Aside from raffinose, other sugars, including sorbitol and fructose, can also be causes of foul flatulence, according to the International Foundation for Gastrointestinal Disorders. Sorbitol can be found in fruit, sugar-free candy, and artificial sweeteners. Fructose is found in fruit, soft drinks, onions, wheat, artichokes, and pears.

Keeping a food diary can help determine if sorbitol or fructose 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," Dr. Raymond remembered.

05 of 07

Constipation

Fried foods, processed meats, baked goods, and high-fat foods aren't necessarily gas-producing. But these low-fiber foods may make it difficult to move your bowels, according to Johns Hopkins Medicine. That's when things get smelly.

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.

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

06 of 07

Medicines

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," Dr. Bazaldua said.

But don't use Beano with drugs in this class, Dr. Bazaldua said, because it can affect how well the medicine works.

07 of 07

Infection or Disease

Smelly gas alone is usually nothing to worry about, Dr. Gandolfo said. But certain conditions can cause a change or increase in gas, according to the National Institute of Diabetes and Digestive and Kidney Diseases.

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. 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
Celiac disease (an immune reaction to gluten)
C. difficile (a bacterial infection that causes diarrhea)

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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