7 Possible Reasons for Smelly Farts

If you notice your farts smell like rotting eggs, this is most likely because of hydrogen sulfide. This gas is produced by the bacteria that break down food in your digestive system. When it combines with other gases your body creates, the result is the unpleasant rotten egg, or sulfur, smell.

Gas is a healthy, normal byproduct of digestion, and most people pass gas 13–21 times a day. Sometimes, smelly farts happen if too many bacteria rapidly break down undigested food in your large intestine. Other times, excess smelly farts may signal a health condition.

Here are seven of the most common reasons your farts might be smelly and ways you might reduce the odor. 

1. Bacteria Build-Up

Your large intestine contains bacteria, fungi, and viruses, known as your gut microbiome. Those microbes aid digestion by breaking down the carbs you eat. That process results in gas.

During digestion, your gut bacteria produce sulfur-containing compounds like hydrogen sulfide that create a stench in your gas, Patricia Raymond, MD, an assistant professor of clinical internal medicine at Eastern Virginia Medical School, told Health.

The number and type of microbes in your gut may affect gas symptoms, including how odorous your farts are. 

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

2. Food Intolerances 

The foods you eat can influence the bacteria that live in your colon, which then affects the smell of your farts, explained Dr. Gandolfo. 

For example, some people cannot fully digest sugars like fructose and lactose. Fructose is a sugar in fruits and honey. Lactose is a sugar in dairy products like cheese, ice cream, milk, and yogurt.

Those sugars pass through your small intestine unabsorbed if you are fructose or lactose intolerant. Your gut bacteria will then break down those sugars in your large intestine, producing excess gas.

3. High-Fiber Foods

Eating too much fiber produces excess gas, which may increase the amount of smelly farts.

Fiber is a carb found in fruits, vegetables, and whole grains that your body cannot fully digest. Fiber aids digestion and helps regulate your bowel movements. 

The National Institutes of Health (NIH) advises 25 grams of fiber per day for women and 38 grams for men. Though, most people in the United States do not eat enough fiber. Slowly increasing your fiber intake helps prevent excess gas. 

4. Constipation

Gas can result from constipation, when you have a hard time pooping. Stool takes longer than usual to pass through your 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," said Dr. Gandolfo. 

Drink plenty of water and exercise regularly to keep things from getting backed up, added Dr. Gandolfo.

5. Medicines

Some medicines can affect your gut and increase gas production. The reasons they cause gas differ by the drug.

How often medicine makes you pass smelly gas varies from person to person, Oralia Bazaldua, PharmD, a professor of family and community medicine at UT Health San Antonio, told Health.

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

6. FODMAPs

The small intestine does not readily absorb high-FODMAP foods, which may cause excess smelly farts. FODMAPs are fermentable short-chain carbs, including fiber, sugars, and starches.

FODMAPS include:

• Fructans: Garlic, onions, and wheat
• Fructose: Fruits (e.g., bananas, grapes, and peaches), honey, and high-fructose corn syrup
• Galactans: Beans and legumes
• Lactose: Dairy products (e.g., cheese, milk, and yogurt)
• Polyols: Pitted fruits (e.g., cherries, apples, and avocados)

Those carbs collect water as they move through your gut. Bacteria quickly break down those carbs in your bowel, resulting in gas. 

FODMAPs are likelier to cause bloating, diarrhea, and gas in people with irritable bowel syndrome (IBS) than others. IBS affects the large intestine and causes changes in bowel habits like constipation, diarrhea, or both.

7. Infections and Diseases

Smelly gas is usually nothing to worry about, said Dr. Gandolfo. Still, certain health conditions may cause a change or increase in gas.

Abdominal pain that does not go away after passing gas or having a bowel movement, or changes in your bowel habits, may signal a health condition.

How To Reduce the Smell of Farts

A healthcare provider may advise adjusting your diet and swallowing less air to help alleviate gas symptoms, including smelly farts. For example, avoiding certain foods or eating small, more frequent meals may reduce the number of bacteria that produce odor. 

Certain medicines and supplements may help, too. Consult a healthcare provider before starting a new over-the-counter (OTC) medicine or supplement.

Change Diet and Eating Habits

Try to avoid foods that may cause smelly gas. Keeping a food diary can help you figure out what foods affect you.

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

Find ways to swallow less air between and during. For example, avoid drinking beverages with a straw or sit down while you eat. Eating smaller, more frequent instead of three large meals may help, too.

Over-the-Counter Medications

A few over-the-counter (OTC) medicines may relieve gas. For example, Beano helps break down some complex carbs. Pepto-Bismol (bismuth subsalicylate) may reduce smelly gas.

Check with a healthcare provider before using any of those medicines. They can ensure that those medicines are safe to take, considering your health or other drugs you may take.

A Quick Overview

Bacteria break down carbs in your large intestine, releasing a gas that smells like rotten eggs. You may have excess smelly farts if more bacteria rapidly break down carbs in your large intestine, which releases more gas than usual.

People who are fructose or lactose intolerant, are sensitive to high-FODMAP foods, or eat too much fiber may notice increased gas symptoms. Certain medicines, constipation, and some GI-related illnesses may also increase gas.