Why Does My Poop Smell Like Onions?

Naturally, poop never smells good. However, sometimes your trip to the washroom can end up smellier than you would ordinarily expect. If your poop produces an onion-ish smell, you may freak out and wonder if it’s occasioned by a previous salad you took or something more threatening like colon cancer. People diagnosed with colon cancer may have blood in their stool that smells like onion. But before you freak out, let’s look at some possible causes of the onion smell in poop.

Poor Digestion

If you’ve been eating a lot of onions, they may not be digesting properly. People with irritable bowel syndrome (IBS) may find they can’t digest onions properly. If that is the case and you’re relatively young (below 55 years), then you can rule out anything serious. And if you don’t spot blood in your poop, you should not worry about colon cancer or other serious issues.   

Fortunately, there are several measures you can take to address IBS and prevent such issues from occurring every time you take some salad. But if the symptoms persist, you may need to get an accurate diagnosis from your doctor.  

Here are some things you could do:

  • Monitor your stress levels to determine whether stress triggers your symptoms.
  • Create a food diary to help you figure out all the foods that cause you problems.
  • Don’t frequently examine your bowel movements, as it may cause you unnecessary anxiety.

More importantly, get a professional diagnosis to help to clear up any health anxiety and eliminate the problem. When you get an accurate IBS diagnosis, you may clear up your health anxiety and improve symptoms.


Other Causes of Bad Poop Smell

Besides IBS and colon cancer, your stool might smell bad due to the following:

1. Taking a Lot of Alcohol

Bathrooms often smell very bad after a night out. Alcohol can affect other organs like the stomach, small intestines, and large intestines by affecting their ability to move things around. Drinking a lot of alcohol can also cause you to have diarrhea, and your stool can smell different from what it used to smell. If you consume more sulfates, it is easier for bacteria in your body to make sulfide gas, which smells a foul stench whenever you visit the bathroom.

2. Taking Certain Medications

Some medications can change the smell of the stool. Certain drugs and hormones can interact with the bacteria in your body and cause nutrient malabsorption and other symptoms like thick, stinky stool.

3. Having a High-Fat Diet

If you eat a lot of fat, your system may be overwhelmed. Your system may be unable to break down the fat you consume. So the fat that remains in your body in its unabsorbed state gets passed to your colon. It can cause very stinky diarrhea with a very high-fat content, known as steatorrhea.


4. Gut Disease or Infection

Steatorrhea causes loose and large stools that are very difficult to control. In some cases, the stools are paler and produce a smell that seems stronger than normal. If your stool is becoming loose and smells like rotten oil, but you still have severe steatorrhea, something more serious may be going on. Having a lot of blood in your stool could signal that you have Celiac disease or pancreatitis.

Steatorrhea causes very strange-looking and very foul-smelling stools. In some cases, bowel inflammations can cause a lot of diarrhea. It is common for infection of the intestines, like a bacterial infection called Clostridium difficile, to cause very bad-smelling diarrhea. It is always a good idea to visit your healthcare provider for a proper diagnosis.

5. Lactose Intolerance

Lactose intolerance occurs when the body cannot break down the lactose found in milk. You can experience different digestive problems if you are lactose intolerant, like diarrhea and gas. If you have lactose intolerance, it will cause you to run to the bathroom frequently if you eat dairy foods. It may even cause nasty, stinky, painful diarrhea.


6. Taking Certain Supplements

Taking certain supplements may improve your health, but they can also cause your stool to smell horrible. Some supplements, like chondroitin, and garlic, glucosamine, are high in sulfate. The bacteria in the colon convert these to sulfide gases. Sulfide gas stinks when it’s breathed in and can cause a horrible odor in the bathroom.

7. Eating a Lot of Sulfur-rich Foods 

Sulfur-rich food sources include cruciferous vegetables (think broccoli and cauliflower), dairy products, and some types of meat. But just like drinking alcohol or taking sulfate-based medications can cause your stools to smell, sulfur-containing foods can also cause your poop to smell. So even though sulfur-rich foods are good for you, they can cause your gas to be very smelly.

When To See a Doctor

If your stool smells horrible, the problem might be serious. A stool that looks bloody may also indicate that you have a serious problem. Call your doctor immediately. Blood that gets into your GI tract is usually rotten-smelling and may appear dark in color. If you feel like something is stuck in your stool, talk to your healthcare provider. 

If your stool is unusually thick or greasy, you may want to talk to your doctor about a condition of your intestines (like celiac disease or a GI infection). It’s worth calling or driving to your doctor if you notice that your stool size, color, or frequency is changing a lot or if you have pain while you poop.


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