Dark and hard stools may be caused by constipation while eating food such as duck blood, or it may be caused by enteritis, gastrointestinal bleeding and other diseases. Constipation patients, food residues in the intestinal tract for a long time accumulation, can not be discharged from the body in a timely manner, water is absorbed by the intestinal tract will appear dry and hard stools. Food containing spinach, black rice, taking iron supplements to replenish blood may appear black stool, if at the same time there is constipation, there may be a temporary black hard stool, stop eating the food, after a few days can be relieved. Enteritis leads to intestinal mucosal damage and bleeding, or gastric cancer, cirrhosis, esophagogastric fundus varices and other gastrointestinal bleeding, hemoglobin in the blood in the gastrointestinal tract, chemical reaction, the formation of iron sulfide, the stool will become black, if the patient usually drink less water or accompanied by constipation, there will be a hard and black stool. If there are black hard stools, after excluding diet and other physiological reasons, there may be other reasons, you should consult a doctor in time to find out the cause, and under the guidance of the doctor for treatment.