Black and green stool usually comes from something you ate, drank, or swallowed as a supplement. Dark leafy greens, iron pills, bismuth-based medicines like Pepto-Bismol, and certain foods like blueberries or black licorice can all shift your stool into that dark green-to-black range. Less commonly, the color signals bleeding in the upper digestive tract, which looks and smells distinctly different from food-related discoloration.
How Stool Gets Its Normal Color
Your liver produces bile, a greenish fluid that helps digest fats. When bile first enters your small intestine, it’s bright green. As it travels through the digestive tract, bacteria in your colon break it down into a pigment called stercobilin, which gives stool its typical brown color. This chemical transformation takes time. If anything disrupts the process or adds its own pigment along the way, the end result changes color.
Why Stool Turns Green
Green stool almost always means one of two things: you ate something green, or food moved through your intestines faster than usual.
When digestion speeds up, bile doesn’t have enough time to fully break down from green to brown. Diarrhea is the most common reason for this rapid transit. Infections from bacteria like Salmonella or E. coli, viruses like norovirus, or parasites like Giardia can all cause a “gush” of unabsorbed bile that turns stool bright or dark green.
On the dietary side, large servings of spinach, kale, or other leafy greens contain enough chlorophyll to tint stool green on their own. Green food coloring found in drink mixes, ice pops, and candy does the same thing. Iron supplements are a particularly common culprit. They can produce stool that ranges from dark green to nearly black, depending on the dose and your individual digestion.
Why Stool Turns Black
Black stool falls into two categories: harmless discoloration and internal bleeding. The distinction matters, and fortunately, it’s not hard to tell them apart.
The harmless causes include iron supplements, bismuth subsalicylate (the active ingredient in Pepto-Bismol and Kaopectate), activated charcoal, black licorice, blueberries, and blood sausage. Bismuth reacts with small amounts of sulfur in your saliva and digestive tract to form a black compound. This is temporary and harmless, though it can also darken your tongue. Iron supplements oxidize in your gut, producing a similar dark coloring.
The concerning cause is bleeding in the upper digestive tract, such as in the stomach or the first section of the small intestine. When blood is digested as it passes through the gut, it turns black and tarry. This condition is called melena.
How to Tell Melena From Harmless Discoloration
Three features distinguish melena from food- or supplement-stained stool:
- Texture: Melena is sticky and tarry, almost like roofing tar. Stool darkened by food or supplements has a normal consistency.
- Smell: Digested blood produces a distinctly strong, offensive odor that’s noticeably worse than usual. Stool darkened by diet or medication won’t have this characteristic smell.
- Color quality: Melena is jet black. Diet-related darkening often has a greenish tint or looks more like a very dark brown.
If your stool is black, tarry, sticky, and unusually foul-smelling, that combination points toward bleeding rather than something you ate. A doctor can confirm with a simple chemical test that detects hidden blood in a stool sample.
Black and Green Together
Seeing both colors at once is common and usually has a straightforward explanation. Iron supplements are the classic cause of stool that looks simultaneously dark green and black. Eating a large salad followed by Pepto-Bismol could produce a similar mix. Rapid digestion from a stomach bug can push green bile through quickly while also creating darker, more concentrated stool.
The combination of black and green, on its own, is not more alarming than either color alone. What matters is whether the stool has the tarry, sticky, foul-smelling qualities of melena or simply looks unusual in color.
How Long the Color Change Lasts
Any food- or supplement-related color change should clear up within a few days once you stop eating the offending food or taking the supplement. If you’re on iron supplements and the color bothers you, know that it’s expected and will persist as long as you keep taking them.
If you stop all possible dietary causes and your stool remains black and tarry after several days, that’s worth investigating. The same applies if you notice other symptoms alongside the color change: lightheadedness, fatigue, abdominal pain, or vomiting that looks like coffee grounds. These suggest blood loss rather than pigment.
Stool Color in Newborns
If you’re a new parent noticing blackish-green stool in your baby’s diaper, that’s likely meconium, which is a baby’s first stool. Meconium is thick, sticky, and blackish-green, made up of materials the baby swallowed in the uterus: water, cells, mucus, and fine hair. Most babies pass meconium within the first 48 hours of life, and their stool gradually transitions to yellow or brown over the following days. If a newborn hasn’t passed meconium within 48 hours, it could indicate an intestinal issue that needs evaluation.