Why Is My Poop Black? 9 Most Common Reasons

Black poop in the toilet can be distressing, but there are lots of reasons why black stool happens, and not all of them are serious.

roll of toilet paper on black backgroundJozsef Zoltan Varga/Getty Images

When black poop happens, family practitioners like Christine Traxler, MD, and pediatricians like Alison Mitzner, MD, are often the first ones their patients call. As Dr. Traxler puts it, “There’s been lots of scary stuff in the media about black stool being a sign of cancer or other serious health problems. So it’s often where my patients’ minds go.”

That being said, Dr. Traxler, a family medicine doctor in Le Sueur, Minnesota, welcomes those calls because although most of the time black stool doesn’t signify anything serious, that’s not always the case. And as Dr. Mitzner notes, it’s always a good idea to check with your doctor to be sure. (Don’t miss these 12 insider tips for choosing the best primary care physician.)

What’s a normal poop color?

As a general rule, stool is brown in color, but normal stool comes in all different shades of brown, yellow, and green, says Dr. Mitzner, who is based in New York. That’s because the color of stool is a function of what you eat and what’s going on in your digestive tract. Just because your stool happens to come out some other color doesn’t mean you’re dealing with a serious condition.

What it might signify is that as your poop made its way through your gastrointestinal (GI) tract, something stained it another color, explains Jeffery Nelson, MD, the surgical director at The Center for Inflammatory Bowel and Colorectal Diseases at Mercy Hospital in Baltimore. For example, your poop may come out vivid green because of something you ate (or any of these other reasons why your poop might be green).

So, let’s examine what may be going on to turn your stool black.

You eat black licorice or other dark-colored foods

Sometimes patients present with black stool simply because they’ve been eating dark-colored food or activated charcoal. The number one culprit is black licorice. But blueberries, beets, and grape juice can also cause stool to appear black. What food alone won’t do is cause the stool to appear tarry, which is a sign of melena, a substance that results from the breakdown of red blood cells. (Read on for more details.)

You’re taking Pepto-Bismol

Black stool can also be caused by ingesting bismuth, the active ingredient in Pepto-Bismol, according to Harry Thomas, MD, a gastroenterologist in Austin, Texas. (Check out these bizarre side effects of common medications.) Black stool caused by bismuth also does not appear “tarry” or has the characteristic odor of melena.

You’re taking iron supplements

Another completely benign cause of black stool is the taking of iron supplements. “Iron consumption often causes black stools because not all of the iron is absorbed,” Dr. Traxler explains. And the iron that isn’t absorbed undergoes chemical changes in the gut, adds Dr. Nelson, such that by the time it leaves the body in your stool, it’s stained your stool black.

But again, as Dr. Thomas notes, black stool caused by iron supplementation does not cause tarry stool. (Here are some foods you can eat to boost your iron levels without the need for supplements.)

You swallowed blood

Swallowing blood from a bloody nose can cause stool to appear black, according to Dr. Traxler. So can swallowing blood from any injury to the mouth, nose, or throat. In addition, sometimes vomiting can cause a tear in the esophagus that can lead to bleeding that turns the poop black. (Here are 10 toilet habits you need to stop doing right now.)

You have bleeding in the upper GI tract

When there is bleeding in the upper GI tract (which includes the stomach, esophagus, and duodenum, the section just after the stomach), the red blood cells break down and become black in appearance, explains colorectal surgeon, Lynn O’Connor, MD, director of Colon & Rectal Surgery of New York and section chief of colon and rectal surgery at Mercy Medical Center and St. Joseph Hospital in New York.

But when upper GI bleeding is the cause of black stool, the black is typically tarry in appearance, Dr. O’Connor explains. This is what doctors refer to as melena and it can often have a particularly noxious odor due to the breakdown of red blood cells.

If you have black stools, bleeding in the GI tract is by no means the default explanation. Even when there is bleeding in the upper GI tract, it doesn’t automatically mean cancer, which, as Dr. Traxler notes, is what most of her patients tend to worry about.

There are many conditions that can cause bleeding in the upper GI tract. Here are some of the most common:

You’re on blood thinners or other medications

Taking blood thinners can also cause black stool, notes Kumar Desai, MD, gastroenterologist, hepatologist, and pancreaticobiliary specialist in Thousand Oaks, California. While it’s a relatively benign reason, it’s still something you should bring up to your doctor because it is indicative of bleeding from the upper GI tract. Aspirin and other non-steroidal anti-inflammatory medications (NSAIDs) can also cause black stools due to bleeding in the GI tract. (Don’t miss these 21 health secrets your gut is trying to tell you.)

You may have a polyp or a tumor in the upper GI tract

Any kind of abnormal growth in the GI tract, whether benign or cancerous, can weaken the lining, which can lead to bleeding. As stated above, bleeding that stains stool in the upper GI tract will appear black by the time the stool makes its way out of the body. Something that probably is not causing your stool to be black, however, is a tumor or growth in the colon or large intestine (again, whether cancerous or not).

As Dr. Traxler points out, when tumors and other growths in the lower part of the intestine cause bleeding, the blood does not remain in the GI tract long enough to turn black (and remains red). (Here are four signs you might need an early screening for colon cancer.)

You may have a peptic ulcer

Peptic ulcers are sores in the lining of your upper GI tract. They’re usually caused by bacteria such as H. pylori or by long-term use of NSAIDs. Peptic ulcers can bleed, thus turning poop black by the time it exits the body. However, if you have a peptic ulcer, you will probably have some other symptoms such as a swollen belly, indigestion, burning pain in the belly, and vomiting that looks like coffee grounds. (Here are 10 ulcer symptoms you should never ignore.)

You may have esophageal varices

Esophageal varices are like varicose veins, but in your esophagus. They can cause bleeding in the upper GI tract and black stool as a result. Esophageal varices are usually related to cirrhosis of the liver, which is a chronic liver condition, and black stool would rarely be your first clue that you were suffering from it. Other symptoms of liver disease and esophageal varices include jaundice (yellowing of the skin and eyes), vomiting blood, dizziness, a swollen belly, and more.(Here are 13 ways you might be hurting your liver without realizing it.)

Again, upper GI bleeding is just one possible cause of stool appearing black, and fortunately, its presence can be confirmed relatively easily by having a stool sample examined for the presence of blood. “The test takes mere seconds,” Dr. Traxler notes, “and it can help decide if more testing needs to be conducted.”

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Lauren Cahn
Lauren Cahn is a New York-based writer whose work has appeared regularly on Reader's Digest, The Huffington Post, and a variety of other publications since 2008. She covers health, fitness, yoga, and lifestyle, among other topics. An author of crime fiction, Lauren's book The Trust Game, was short-listed for the 2017 CLUE Award for emerging talent in the genre of suspense fiction.