Changes in Stool Appearance: What Color, Shape and Consistency Mean

What normal stool looks like

There is no single "normal". Healthy bowel movements range from three a day to three a week. Stool is usually brown because of bilirubin, a pigment from broken-down red blood cells that the liver puts into bile. Bacteria in the gut change it from green-yellow to brown as it passes through.

Doctors describe consistency with the Bristol Stool Chart:

What matters most is a change from your own normal, especially one that lasts or comes with other symptoms.

Stool color changes and what they mean

Black stools

True melena is black, sticky and tar-like, with a strong, distinctive smell. It means blood has been digested on its way down from the esophagus, stomach or duodenum. Stomach ulcers, often caused by NSAIDs such as ibuprofen or aspirin, are the classic cause. Black stools from iron or bismuth are usually dark green-black, formed and not sticky. Read more about melena.

Red or bloody stools

Bright red blood on the paper or on the surface of the stool usually comes from near the anus, most often hemorrhoids or an anal fissure. Blood mixed through the stool, dark red blood or clots come from higher up. Any recurring blood in the stool needs assessment, especially after age 45 or with a change in habit.

Pale or clay-colored stools

When bile cannot reach the gut, stools lose their brown color and turn pale gray, putty or white. This happens when gallstones, inflammation of the liver or a tumor block the bile ducts. It often comes with dark, tea-colored urine and jaundice. Pale stools that last more than a day or two need a doctor. In a newborn or young baby, pale stools are urgent because they can signal biliary atresia, which is treatable only if caught early.

Changes in consistency and form

Loose or watery stools

Short bouts of diarrhea are usually caused by viral gastroenteritis, food poisoning or a change in diet. Diarrhea lasting more than 4 weeks is chronic and has a different list of causes: irritable bowel syndrome, celiac disease, lactose intolerance, microscopic colitis, inflammatory bowel disease, an overactive thyroid and medicines such as metformin.

Hard, lumpy stools

Hard stools that are difficult to pass mean constipation. Common causes are low fiber, too little fluid, inactivity, iron supplements, opioid painkillers and ignoring the urge to go. New, persistent constipation in someone over 45 is worth discussing with a doctor.

Pencil-thin or narrow stools

Occasional thin stools are normal and often due to straining or IBS. Stools that are consistently narrow over weeks can mean something is narrowing the lower bowel, such as a tumor or a stricture, and should be checked.

Floating, greasy, foul-smelling stools

Stool that floats is usually just full of gas from fiber or fizzy drinks. Stool that is pale, oily, bulky, smells very bad, leaves an oily film and is hard to flush is called steatorrhea. It means fat is not being absorbed. Causes include celiac disease, chronic pancreatitis, pancreatic insufficiency and cystic fibrosis.

Mucus in the stool

A little clear mucus is normal. More mucus, especially with urgency and cramps, is common in irritable bowel syndrome. Mucus with blood suggests inflammation, as in ulcerative colitis or an infection.

Undigested food

Seeing corn, seeds or vegetable skins is normal, because humans cannot digest some plant fiber. Undigested food with diarrhea and weight loss suggests malabsorption.

Causes grouped by how serious they are

Common and usually harmless

• foods, drinks and food coloring
• iron, bismuth, antibiotics and other medicines
• a short stomach bug or food poisoning
• travel, stress, and changes in routine or diet
• irritable bowel syndrome

Conditions that need medical care

• inflammatory bowel disease: Crohn's disease and ulcerative colitis, with bloody diarrhea, urgency, pain and weight loss
• celiac disease and other causes of malabsorption
• peptic ulcers and other sources of gut bleeding
• liver, gallbladder and pancreatic disease
• persistent infections such as Giardia or C. difficile after antibiotics

Serious causes to rule out

• colorectal cancer, which can cause a change in bowel habit, blood in or on the stool, anemia and weight loss
• cancers of the pancreas or bile ducts, which cause pale stools and jaundice

Symptoms that change the picture

How doctors work out the cause

Expect questions about how long the change has lasted, what you eat and drink, medicines and supplements, travel, family history of bowel cancer or IBD, and any blood, pain or weight loss. Bringing a photo or a description using the Bristol scale helps.

Tests may include:

• Blood tests: blood count for anemia, inflammation markers, liver tests, celiac antibodies, thyroid function.
• Stool tests: infection screens, fecal calprotectin (raised in IBD, usually normal in IBS), fecal immunochemical test (FIT) for hidden blood, and fecal elastase for pancreatic function.
• Colonoscopy or upper endoscopy: to look directly at the bowel lining and take biopsies.
• Imaging: ultrasound or CT for the liver, gallbladder and pancreas.

What you can do

• Look back over the last 48 hours: beets, colored drinks, iron and bismuth explain many color changes.
• For short bouts of diarrhea, drink plenty of fluids. Oral rehydration solution is best for heavy losses.
• For hard stools, add fiber gradually (fruit, vegetables, oats, beans), drink enough to keep urine pale, and stay active.
• Keep a simple diary of stools, food and symptoms if changes come and go. It speeds up diagnosis.
• Take part in bowel cancer screening from age 45 in the US, or as advised where you live.

Do not keep taking over-the-counter antidiarrheal medicine if you have blood in the stool or a high fever, because it can make some infections worse.

Frequently asked questions

When should I worry about a change in stool color?

Worry about black tarry stools, red or maroon stools, or pale clay-colored stools that are not explained by food or medicine. These can mean bleeding or a blocked bile duct. Any change that lasts more than 3 weeks, or comes with weight loss, pain or tiredness, also needs a doctor.

What do changes in bowel movement and form mean?

A short change in form, such as a few days of loose or hard stools, usually reflects diet, a stomach bug, stress or travel. A lasting change in consistency, frequency or shape can point to IBS, celiac disease, inflammatory bowel disease, thyroid problems or, less often, colorectal cancer.

Why is my poop green?

Green stool usually comes from leafy vegetables, green coloring, iron supplements, or diarrhea moving stool through too fast for bile to turn brown. It is rarely a concern unless it comes with ongoing diarrhea, fever or pain.

Can stress change how your stool looks?

Yes. Stress speeds up or slows gut movement through the gut-brain connection, causing looser or harder stools, more mucus and cramps. It is a common trigger in IBS. Stress does not cause black, bloody or pale stools, so those need checking.

What does it mean if stool floats?

Usually it means extra gas from a high-fiber diet or fizzy drinks, which is harmless. Floating stool that is also greasy, pale, very smelly and hard to flush can mean fat malabsorption from celiac disease or a pancreatic problem.

Are thin stools a sign of cancer?

Occasionally thin stools are normal. Stools that stay narrow for weeks, especially with blood, a change in habit, or a feeling of incomplete emptying, can be caused by a growth in the lower bowel and should be investigated, usually with a colonoscopy.