Poop is brown because of a pigment called bilirubin, which your liver produces as it breaks down old red blood cells. Bile carries bilirubin into your intestines, where bacteria chemically transform it into the brown pigment you’re used to seeing. When something disrupts that process, whether it’s a food you ate, a medication, or a health condition, your stool shifts color. Most of the time it’s harmless and temporary, but certain colors deserve attention.
How Bile Creates the Normal Brown
Your liver constantly filters old red blood cells out of your bloodstream, producing bilirubin as a waste product. This bilirubin gets mixed into bile, a digestive liquid that your liver sends to your gallbladder for storage. After you eat, bile releases into your small intestine to help break down fats. As bile travels through your intestines, gut bacteria convert bilirubin through a series of chemical changes that gradually turn it brown. The longer stool sits in your colon, the more complete that conversion becomes.
This is why transit time matters so much. When food moves through your system at a normal pace, bile has enough time to fully break down, and you get the expected brown result. Speed things up with diarrhea, and the bile doesn’t fully convert. Slow things down, and the color can darken. Nearly every stool color change traces back to one of three things: what went in (food or medication), how fast it moved through, or whether bile reached the intestines at all.
Green Stool
Green is probably the most common color shift people notice, and it’s almost always diet or transit speed. Chlorophyll, the pigment that makes vegetables green, passes through your digestive system largely intact. Eating a lot of broccoli, kale, spinach, or other leafy greens can tint your stool visibly green.
The other major cause is speed. Bile starts out yellow-green when it first enters your intestines. If diarrhea pushes everything through too quickly, bacteria don’t have time to finish converting the bile pigments to brown, so what comes out still looks green. This can happen with stomach bugs, food poisoning, or parasitic infections like giardia. Some medications cause green stool the same way: they upset the stomach and trigger bile-rich diarrhea. People who’ve recently had their gallbladder removed sometimes notice green stools for the same reason, as extra bile temporarily floods the digestive tract.
Red Stool
Red stool gets people’s attention fast, but it’s worth pausing before panicking. Beets, red food dye, tomato soup, red velvet cake, and even blackberries can all turn stool pink or red. The antibiotic cefdinir, commonly prescribed for ear infections, reacts with iron in the gut to create a red or maroon discoloration that looks alarming but is completely harmless.
When the red color comes from actual blood, the shade and pattern offer clues. Bright red streaks on the surface of stool typically point to bleeding in the lower digestive tract: hemorrhoids, small tears in the anal lining (fissures), or inflammation in the colon. Blood mixed throughout the stool, or stool with visible clots and mucus, can signal inflammatory bowel disease or, less commonly, colorectal cancer. Bacterial infections from E. coli or C. diff can also cause bloody diarrhea. If you haven’t eaten anything red recently and you’re seeing red in the toilet, that’s worth a call to your doctor, especially if it happens more than once.
Black or Very Dark Stool
Black stool has two very different explanations, and telling them apart isn’t difficult once you know what to look for. The harmless version comes from iron supplements, bismuth subsalicylate (the active ingredient in Pepto-Bismol), or foods like black licorice and blueberries. These stain the stool dark but don’t change its texture or smell.
The concerning version is called melena. It happens when bleeding occurs high in the digestive tract, in the stomach or upper intestines. As blood passes through the full length of the gut, digestive enzymes break it down, turning it jet black. Melena has a distinctive tarry, sticky consistency and a particularly strong, foul odor that’s noticeably different from normal stool. If your black stool is firm or formed and doesn’t have that sticky, tar-like quality, a supplement or food is the likely culprit. If it’s tarry and unusually foul-smelling, that’s a sign of active bleeding that needs medical evaluation.
Pale, White, or Clay-Colored Stool
This is one color change you should never ignore. Stool gets its brown color from bile, so when stool turns pale, clay-colored, or chalky white, it often means bile isn’t reaching the intestines. The medical term for this is acholic stool.
The most common cause is a blockage in the bile ducts, the tubes that carry bile from the liver to the small intestine. Gallstones, tumors, or inflammation can all create this blockage. When bile can’t flow out, it builds up in the liver and eventually spills into the bloodstream, which is why pale stool often comes alongside jaundice (yellowing of the skin and eyes), dark urine, and itching. One exception: aluminum hydroxide antacids can temporarily lighten stool to white or speckled without any underlying problem. But persistent pale stool, particularly with other symptoms, signals a biliary obstruction that needs prompt attention.
Yellow or Greasy Stool
Occasional yellow stool after a high-fat meal isn’t unusual. But stool that’s consistently yellow, greasy, foul-smelling, and tends to float points to fat malabsorption, a condition where your body can’t properly digest and absorb dietary fat. The undigested fat passes through and gives stool a pale yellow, oily appearance.
Several conditions cause this. Celiac disease damages the lining of the small intestine, reducing its ability to absorb nutrients including fat. Chronic pancreatitis or other pancreatic problems reduce the supply of digestive enzymes needed to break fat down. Bile acid deficiency, whether from liver disease or surgical removal of part of the intestine, has a similar effect. Parasitic infections like giardia can also impair fat absorption temporarily. The weight-loss drug orlistat works by deliberately blocking fat absorption, so greasy yellow stool is an expected (if unpleasant) side effect.
Medications That Change Stool Color
The list of medications that alter stool color is surprisingly long. Iron supplements and Pepto-Bismol are the best-known offenders, both turning stool black. But the range extends further:
- Orange or red: Rifampin (a tuberculosis antibiotic) and some anti-nausea medications
- Green-gray: Certain antibiotics and iron compounds
- Yellow-green: Senna-based laxatives
- Blue: Methylene blue, occasionally used in diagnostic procedures
- White or speckled: Aluminum-based antacids and some antibiotics
These changes are almost always harmless and resolve once you stop the medication. The important distinction is whether the color change showed up right after starting a new drug or supplement. If the timing lines up, that’s your answer. If it doesn’t, or if you have pain, fever, or diarrhea alongside the color shift, the medication may not be the explanation.
Stool Color in Babies
Infant stool goes through a dramatic and completely normal color progression in the first days of life. A newborn’s first stools are black or dark green and sticky. This is meconium, a mix of everything the baby swallowed in the womb, and it clears out within the first two to three days.
After meconium passes, stool transitions to a yellow-green color. Breastfed babies settle into a pattern of seedy, loose, light-mustard-colored stool. Formula-fed babies tend to have firmer stool that’s yellow or tan, sometimes with green hints, roughly the consistency of soft peanut butter. Green stool in babies is also normal and not a cause for concern on its own. The one color that should prompt a call to the pediatrician is white, pale, or clay-colored stool in an infant, as it can indicate biliary atresia, a serious condition where bile ducts don’t develop properly.
When Color Changes Are Worth Tracking
A one-time color change after eating beets or starting a new vitamin is nothing to worry about. The shift should resolve within a day or two once the food or supplement clears your system. Food typically takes 24 to 72 hours to move completely through the digestive tract, so that’s a reasonable window to watch.
Color changes that persist beyond a few days without an obvious dietary explanation deserve attention, especially red or black stool that you can’t trace to food or medication, pale or clay-colored stool at any age, and yellow greasy stool that keeps recurring. Any color change paired with fever, abdominal pain, unexplained weight loss, or diarrhea that won’t resolve adds urgency. The color itself is useful information to share with a doctor, but it’s the combination of color, consistency, and accompanying symptoms that tells the full story.