Blood in the stool almost always has a benign cause, mainly hemorrhoids or an anal fissure, and a small one-off bleed usually stops on its own. Even so, see a doctor if it keeps coming back, lasts more than two or three weeks or comes with changes in bowel habits, tiredness or weight loss, because it is sometimes the first sign of polyps or colorectal cancer. Black, tar-like stools, heavy bleeding or dizziness are reasons to go to the emergency room.
What does the color of the blood in your stool mean?
The color gives a clue to where the blood is coming from, although it does not tell you the cause.
- Bright red, on the toilet paper, in the toilet water or on the outside of the stool. It usually comes from the anus or the lower rectum.
- Dark red or mixed in with the stool. This suggests a source a little higher up, in the rectum or colon.
- Black, sticky, foul-smelling stools, like tar (melena). This is digested blood, usually from the esophagus, stomach or duodenum, for example from an ulcer. See a doctor immediately.
Not everything red or black is blood. Eating a lot of beets or tomatoes can turn stools red, and iron tablets, black licorice or blueberries can make them very dark. If in doubt, a stool test will settle it.
What are the most common causes?
Hemorrhoids are the most common cause. They bleed bright red during a bowel movement, often after constipation, and may itch or be felt as a lump. An anal fissure also causes red blood, but with sharp pain when passing stool that can last for a while afterwards.
Other benign causes are an anal fistula, which usually leaks pus or mucus as well as blood, colon diverticula, which can bleed heavily and painlessly, and gut infections, which cause bloody diarrhea. Inflammatory bowel disease, ulcerative colitis or Crohn's disease, causes bloody diarrhea and pain that keep coming back over weeks.
Blood-thinning drugs such as anticoagulants or aspirin do not cause bleeding on their own, but they make a small lesion bleed more. Tell your doctor if you take them.
Polyps and colorectal cancer are less common causes, but they are the reason unexplained bleeding has to be investigated. They may bleed a little and on and off, sometimes without visible blood, and often come with changes in bowel habits, mucus or narrower stools. We cover the symptoms of tumors in the last section of the bowel in rectal cancer symptoms.
When should you worry about blood in your stool?
Go to the emergency room if you are bleeding heavily or non-stop, if your stools are black like tar or if you feel dizzy. If there is only a little blood but it keeps coming back or lasts more than two or three weeks, make an appointment with your doctor.
Go to the emergency room if:
- you are bleeding non-stop or heavily, for example if the toilet water turns red or you see large clots,
- your stools are black like tar,
- you vomit blood, or
- you feel dizzy, lightheaded or as if you might faint.
Make an appointment with your doctor, not urgently but without putting it off, if the blood keeps coming back or lasts more than two or three weeks, if your stools have changed (softer, narrower, with mucus) over that time, if you have pain around the anus or in the abdomen that does not settle, or if you are more tired than usual or losing weight for no reason. Do the same if close relatives have had colorectal cancer or polyps, even if the bleeding is minor.
Do not assume it is hemorrhoids without being examined, even if you have had them before. Hemorrhoids and a polyp or tumor can occur at the same time.
What tests are done?
First an examination in clinic, which is often enough to find the cause, and if not, a colonoscopy. In clinic we start by asking about the color of the blood, how long it has been happening, the medication you take and your family history, and then we examine the anal area and do a digital rectal exam. Sometimes the anal canal is checked with a small tube (anoscopy). That is often enough to identify causes such as hemorrhoids or a fissure at the same visit.
If the cause is not clear, the bleeding persists or there are other symptoms or risk factors, a colonoscopy is done. It shows the rectum and the whole colon, and polyps can be removed and biopsies taken. If the stools are black, the first test is usually an upper endoscopy (gastroscopy), which looks at the esophagus, stomach and duodenum. A blood test shows whether there is anemia.
In Madrid we see these patients at the coloproctology clinic at Memorial Publio Cordón Hospital.
The fecal occult blood test is used for colorectal cancer screening in people without symptoms. If you can already see blood, it adds nothing: what you need is an examination.
Frequently asked questions
Is it serious to see blood in your stool once?
A small one-off bleed is not usually serious and is often due to hemorrhoids or a fissure. If it keeps coming back, lasts more than two or three weeks or comes with other symptoms, get it checked.
What does bright red blood on the toilet paper mean?
It usually comes from the anus or the lower rectum, most often from hemorrhoids or an anal fissure. Even so, if it keeps happening a specialist should examine you.
Why are my stools black?
It can be due to iron tablets or some dark foods. If they are black, sticky and foul-smelling like tar, they may mean bleeding from the stomach or duodenum, and you should see a doctor immediately.
Can blood in the stool be cancer?
It can, although that is not the most common cause. That is why bleeding that keeps coming back or has no clear cause is investigated, usually with an examination and, if needed, a colonoscopy.
Can hemorrhoids bleed a lot?
They can stain the paper or the toilet water quite a lot, which is frightening. If the bleeding does not stop, is heavy or there are large clots, go to the emergency room.
I take anticoagulants and see blood in my stool. What should I do?
Do not stop the medication on your own. Talk to your doctor: anticoagulants make a lesion bleed more, but the lesion still has to be found. If the bleeding is heavy, go to the emergency room.
References
- NHS. Bleeding from the bottom (rectal bleeding).
- MedlinePlus, U.S. National Library of Medicine. Black or tarry stools.
This information is for general guidance and does not replace an assessment by your doctor.