A cup of coffee can get your body's motor running to start the day or provide a boost when you're sitting through a dull, afternoon meeting at work. In addition to helping you feel mentally alert, coffee can affect the other end of your body. The drink's caffeine is a known bowel stimulant, but it's unlikely to cause you to have bloody or black bowel movements.
Caffeine and Peristalsis
Drinking coffee can result in frequent trips to the bathroom, as the drink's caffeine leads to the increased contraction and relaxation of the muscles in your colon. This phase, known as peristalsis, causes stools to pass through your system faster than usual. The quickened peristalsis means your colon won't absorb as much water from your stool, which can result in stools that are looser than usual.
Bloody and Black Stools
Despite coffee's ability to speed up the functioning of the bowels, bloody or black stools are often a symptom of something more serious than caffeine use. If you see bright red blood in your stool, you might have issues such as bleeding in your digestive tract, hemorrhoids, anal fissures or inflammatory bowel disease, according to MedlinePlus. Black stools can result from issues such as a bleeding ulcer or gastritis.
Diet Can Change Stool Color
Although coffee is unlikely to change the color of your stools, it's possible to experience black or reddish stools because of your diet. Dark-colored foods such as blueberries and black licorice can give your stool a black appearance, as can certain types of medications. A diet high in red-colored foods such as beets and tomatoes can also result in red stools. In these cases, however, the change in your stool color isn't cause for concern.
Speak to Your Doctor
Because black or bloody stools can be associated with serious health issues, MedlinePlus recommends contacting your family doctor if you see blood, or note changes in your stool color. The organization recommends scheduling an appointment even if you suspect the blood is from hemorrhoids. If you're a parent, be vigilant about tracking the color of your children's stool; MedlinePlus reports that blood in a child's stool can be the result of constipation, but it is still worth mentioning to your child's doctor.