What Could Cause Loose, Dark Stools?

Bowel habits may be embarrassing to talk about, but stool provides important health clues — and your doctor needs to know if something's not quite right. In general, stool should be medium to dark brown with a soft to firm texture. When stool becomes dark and loose, it's important to take notice. While certain dietary habits or medications could be to blame, these stool abnormalities may indicate something more serious.

Bowel movements are a completely normal body function, and your stool provides helpful clues to your digestive health. The frequency of your bowel movements and the consistency and texture of your stool can change based on your fiber and fluid intake. Also, stool color can be influenced by the pigments in fruits, vegetables -- and the dyes added to foods.

What's considered normal varies person to person, but in general stool should be about 75-percent water, with the other 25 percent a mixture of bacteria, undigested food, mucus and bile. More water in the stool makes it too loose, and less water can lead to hard, dry stool.

The color of normal stool is brown, and diet factors and health problems can influence stool color. For instance, pale or yellow stool can be linked to liver disease or infections, and black or red stool is suggestive of blood in the stool.

Dietary fiber adds bulk to intestinal contents and makes fecal matter move more quickly through the intestines. So eating large amounts of fruits, vegetables or other high fiber foods can cause loose stools — but not necessarily dark stools.

Stool can become dark if it's discolored by blue or black food pigments, though. Common foods that cause dark stools include blueberries, black licorice, or blue or black-colored candies.

Certain medications can also cause stools to darken in color. Iron supplements are notorious for causing dark or black stools, although these supplements are more likely to cause constipation than loose stools.

Other medications that can cause dark stools, although not necessarily loose ones, are those that contain bismuth, such as Pepto-Bismol and Kaopectate. Bismuth interacts with sulfur, which is naturally found in your gastrointestinal tract, and forms bismuth sulfide — which has a black color. This stool discoloration is harmless and temporary, and will resolve a few days after you stop taking the medicine.

Stools that are loose and maroon or black are suggestive of bleeding somewhere in the gastrointestinal (GI) tract. If the stool is very dark or black, this can indicate bleeding in the esophagus and stomach, while bright red or maroon stool suggests bleeding in the rectum or large intestine.

A wide variety of conditions and some infections can cause GI bleeding, including liver disease, ruptured blood vessels, ulcers, inflammatory bowel disease, cancer and E. coli infections. So if GI bleeding is suspected, it's important see your doctor right away.

It's important to pay attention to your stool color and consistency, and let your doctor know if your stools change from your usual, normal pattern and appearance. Dark, loose stools may be caused by bleeding into the GI tract, so if the change in stool color and consistency cannot be explained by a dietary reason, talk to your doctor.

Seek immediate medical attention if you are having significant blood in the stool or if the dark stools are accompanied by vomiting, diarrhea, severe pain, weakness or dizziness.

Reviewed by Kay Peck, MPH RD