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Stomach upset is a common problem, and many people will take an over-the-counter remedy to get some relief. Some of the different medicines that can be used for stomach problems include Pepto-Bismol or Kaopectate.

Feeling better typically happens pretty quickly, but later that day or the day after, something really weird happens in the bathroom. The next bowel movement or two show a difference and it could be black stool or maybe even green stool.

Some people might even forget that they took the medication, and then the black stool is really a surprise and a mystery. However, it was actually using Pepto-Bismol or Kaopectate for a digestive problem that has this effect. Green or black stools could be caused by the active ingredient, which is bismuth subsalicylate.

How Bismuth Subsalicylate Works

Pepto-Bismol and Kaopectate (which are two of the well-known brand names, there are also generics or store brands that have similar ingredients) can be found over-the-counter in drug stores.

These medications are often used for an upset stomach, such as after eating too much food, or for mild stomach discomfort from a variety of causes. They're not usually the type of drugs that are prescribed for chronic conditions or ongoing digestive complaints because they may not be effective at treating more serious problems.﻿﻿

People who find themselves taking over-the-counter medications to manage digestive upset for longer than a few days, or who need them several times a month, should contact a physician.

Bismuth subsalicylate is an antidiarrheal drug. In the digestive system it calms inflammation and reduces the amount of water entering the bowel. This can slow down the production of loose stools or diarrhea. It also has antibacterial properties and may inhibit organisms that cause diarrhea and other gastrointestinal symptoms.﻿﻿

A small amount of a substance called sulfur is present both in saliva and in the gastrointestinal tract. Sulfur interacts with bismuth, the active ingredient that's used in Pepto-Bismol. The result is a new substance called bismuth sulfide, which is what causes the stool to turn black.

Stool Color Concerns

The black or green stool after taking bismuth subsalicylate can last for several days, but it is harmless. This change in stool color is not a cause for concern unless you have stopped taking the bismuth subsalicylate and the changes in the stool color persist.

If the black or green color in your stool does not go away a few days after the medicine is stopped, there could be something else going on that's causing the changes.

Black Stool

Black stool can happen for a variety of other reasons, including taking iron supplements and eating certain black, green, or purple foods (like black sandwich cookies). But when it can not be traced to a food or a supplement, a doctor should be consulted because black stools or stools that appear tarry could actually contain blood.

Blood in the stool is always a cause for concern. It is never normal, and it needs to be evaluated by a healthcare provider.﻿﻿

Green Stool

Green stool is also common, especially after eating green or purple foods, However, green stool that continues for a long time could actually be because of a medical problem.

When stool moves through the body too quickly, it doesn't get a chance to be changed to brown, and it stays green. That's why when green stools and diarrhea occur together, it should be discussed with a doctor.

A Word From Verywell

In many cases, thinking back on the foods, supplements, or medicines taken in the last few days can give a clue as to why a stool could be black or another color. Black or green stools that are accompanied by diarrhea, vomiting, abdominal pain or other symptoms should prompt a call to a physician.

Even if it's pretty likely that a stool has changed color because of the bismuth subsalicylate in Pepto Bismol, having these other symptoms is still a reason to contact a physician to make sure there's not a serious condition causing them.