Black Stool: 5 Possible Reasons

Author's Bio: Ashley Oswald, RDN, IFNCP, CLT, LD

If you've noticed black stool, don't freak out—but don't ignore it either.

I'm a Harvard teaching hospital trained gut health dietitian, and I'm about to tell you the five most common reasons this happens—

We’ll cover the harmless causes with simple explanations — and the signs that mean it’s time to see a doctor.

Stick around to the end, where I’ll explain the difference between seeing red blood versus black in your stool — and what each one actually means.

[Video Transcript Below Video - it is the same information]

Reason 1. Iron Supplements

Okay, reason number one.

If you've been feeling exhausted lately and your doctor put you on iron supplement,

Then you might have noticed that your stool turned dark—maybe even black—and now you're wondering if the supplement is doing something bad to your body.

Why Iron Turns Stool Black

Here's what's actually happening:

Your body can only absorb a certain amount of iron at once.

The rest? It has to go somewhere.

As that excess iron moves through your digestive system, it oxidizes—

and that's what creates the dark or black color.

It’s like excess minerals passing through a water filter — what your body doesn’t absorb just flows through and changes the color of the water on the way out.

So if the timing lines up, and this is the only change you’ve noticed, it’s usually nothing to worry about.

But — if the stool looks tarry or you notice other symptoms — always check with your doctor, just to be safe.

Reason 2. Foods That Darken Stool

Now let's talk about the first food that can cause black stool…

And this one catches people off guard alll the time.

You wake up, go to the bathroom, and suddenly you're staring at something that looks terrifying.

Your mind immediately goes to the worst case scenario and wonder if you should go to the ER.

But here's the plot twist:

You ate something completely innocent…

Beets.

Common Foods That Can Turn Stool Black

Beets contain pigments called betalains that don't fully break down during digestion.

Now, beets usually make stool reddish, but very dark beets or concentrated beet juice can sometimes look almost black, especially if you ate a large amount.

And beets aren't the only food culprit.

Lots of blueberries or blackberries can do it too.

Dark chocolate, especially the really high cacao percentage stuff, can darken your stool.

And anything with black food coloring—certain beverages, desserts, or processed foods like black licorice—can absolutely cause this.

Why Timing Matters

Here's the key timing you need to know: transit time, which is how long food takes to move through your entire system

A typical transit time is 24 to 72 hours, with most people averaging 28 to 40 hours.

So that handful of blueberries you ate yesterday?

It's showing up today or even tomorrow.

That blueberry juice you had at dinner two nights ago?

Could be showing up right now.

That's why trying to think of everything you ate in the past day or two can be really helpful.

Activated charcoal will definitely do this too if you've been taking it - this is becoming more popular these days, and cafes are even adding it to smoothies or smoothie bowls.

Just think back to what you've eaten in the last 24 to 72 hours, and you might just solve your own mystery.

But what if it's not food...

Free Gut Health Resource

We’ll get to that in a bit, but first, speaking of figuring out what's going on with your gut—

If you want to know about 5 ways to improve gut health, then I have this free guide for you.

Just click the link to get it delivered to your inbox, to read when you have more time.

Reason 3. Medications Like Pepto Bismol

Okay, back to the non-food reasons for black stool...

Reason three.

You had an upset stomach last night—maybe some nausea, maybe diarrhea—so you took something over the counter to feel better.

And now this morning, black stool.

The medication fixed one problem but created a whole new panic.

Why Certain Medications Cause Black Stool

Here's what you need to know:

Medications containing bismuth subsalicylate—the most common one being Pepto-Bismol—will turn your stool black.

It's a known and completely harmless side effect.

What's actually happening is that the bismuth combines with small amounts of sulfur in your digestive tract to form bismuth sulfide, which is black.

It's just chemistry.

Check the side effects on your medication label, or ask your pharmacist. Once you stop taking medications like Pepto-Bismol, the black stool goes away within a couple of days. Simple as that.

Reason 4. Upper GI Bleeding. When to Worry

But now we need to talk about the serious stuff...

Alright, reason four is where things get real, and you need to pay close attention here.

Because up until now, we've been talking about harmless causes.

But what if your black stool looks different?

What if it's not just dark—it's tarry, sticky, almost like coffee grounds?

And what if it smells worse than anything you've ever experienced?

That's your body waving a red flag.

Melena. What Black Tarry Stool Really Means

This is called melena, and it means there's bleeding somewhere in your upper GI tract—your stomach or upper small intestine.

When blood gets digested as it travels through your system, it turns black and creates that distinctive tar-like texture and that really foul odor.

This is completely different from the black stool we've been talking about from food or supplements.

Possible Causes of Upper GI Bleeding 

And there are a few potential causes of upper gut bleeding.

Regular use of pain medications, such as NSAIDS like ibuprofen or naproxen, can put you at risk,

Or blood thinners like warfarin or aspirin therapy.

If this is you, then pay extra attention to your stool color, and let your doctor know if you notice black stool.

While these medications don't directly cause black stool, they can increase your risk for stomach irritation and ulcers, which CAN cause bleeding.

Further, peptic ulcer disease can cause this, which is basically ulcers in your stomach or duodenum.

These can develop from H. pylori bacteria infections, long-term use of NSAIDs, or even excessive stress combined with other risk factors, such as alongside the medications or alcohol use.

Severe gastritis, which is inflammation of your stomach lining, can also cause bleeding.

Tears in your esophagus from severe vomiting—called Mallory-Weiss tears—or esophageal varices, which are enlarged veins that can happen with liver disease.

Emergency Symptoms You Should Never Ignore

And here's what makes this an emergency—

If you're seeing black tarry stool with that coffee-ground consistency along with ANY of these symptoms, you need to see a doctor immediately:

Abdominal pain that's moderate to severe or getting worse,

Nausea or vomiting especially if there's any blood in your vomit whether it's bright red or looks like coffee grounds,

Feeling weak, dizzy, lightheaded, or unusually fatigued.

These can be signs of significant blood loss.

 Don't wait, don't see if it gets better tomorrow. Go get checked out.

Your doctor can do a simple stool test to check for blood, and if needed, they might do an endoscopy to see exactly what's going on.

The good news is that when caught early, most of these conditions are very treatable.

Ulcers can heal with the right medications, infections can be treated with antibiotics, and even more serious conditions have much better outcomes when addressed quickly.

Modern medicine is excellent for acute issues, while our clinic can help with the chronic gut symptoms and issues using a holistic approach, such as with irritable bowel syndrome, chronic constipation, generalized fatigue, and more.

Reason 5. Chronic Conditions and Disease

So now, reason 5

Let's say you've had digestive issues for a while now—maybe you've been diagnosed with Crohn's disease, ulcerative colitis, or you've been dealing with recurring stomach problems.

Then one day, black stool shows up out of nowhere.

That could be your condition flaring in a way you haven’t seen before.

Inflammatory bowel disease can cause bleeding at different points along the GI tract.


Diverticular disease — where small pouches form in the colon wall — can bleed, too.

Black Stool and Cancer Risk 

And yes, we need to talk about the thing no one wants to think about: cancer.

Here’s something important to understand about bleeding in the digestive system:

Where the bleeding happens usually determines what color you see.

Bright red blood often means bleeding in the lower GI tract — the colon, rectum, or anus.

That’s how most colorectal cancers present, especially when the tumor’s in the rectum or the left side of the colon.

You’d typically see red or maroon blood mixed with stool.

But black, tarry stool — that melena we talked about earlier — usually points to bleeding higher up: in the esophagus, stomach, or small intestine.

That said, colorectal cancer can occasionally cause darker or even blackish stool if the tumor’s on the right side of the colon — like the cecum or ascending colon — and the bleeding is slow and chronic.

In those cases, the blood has more time to partially digest as it moves through your system.

And if that’s paired with other symptoms — like unintentional weight loss, early fullness, loss of appetite, fatigue, or shortness of breath — don’t ignore it. Talk to your doctor.

Final Thoughts

That said, I hope you just ate beets the other day — and now have a better idea of why your stool turned dark without any other red flags.

Chronic constipation is another possible cause. It can lead to fissures or worsen hemorrhoids, which usually show up as bright red blood.

And here’s the kicker: some of the same factors that cause chronic constipation may also raise your risk of colon cancer.

I made a video all about that — how to relieve constipation and reduce your colon cancer risk.

You can check it out here.

Thanks for watching — and as always, stay curious.

If you'd like to explore any of this information further or obtain an individualized nutrition plan, you can schedule an initial appointment at our clinic.  We also take insurance and some of our clients get full coverage, which is great.

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