In this case, if you take some bismuth drugs such as colloidal bismuth pectin, it will usually cause black stools. As for the duration, if you have bowel movements every day and still have black stools after three days, you will have black stools. The stool routine should be checked. At the same time, black stool caused by gastric bleeding is usually like tar, black and shiny. Sometimes eating pig blood will cause black stool.