14 Possible Causes of Blood in Stool

2. Anal Fissures

An anal fissure is a small tear in the thin tissue that lines the anus. It usually develops when a hard or large stool stretches the area during a bowel movement. This tear exposes sensitive nerves, which causes sharp pain during or after passing stool.

You may notice bright red blood on the toilet paper, on the surface of the stool, or in the toilet water. The bleeding is typically small but often occurs alongside a burning or stinging sensation that can last for several hours after a bowel movement. Common causes include constipation, frequent diarrhea, and straining while using the toilet.

Pregnancy, childbirth, and medical conditions that cause intestinal inflammation can also increase the risk of developing anal fissures. If pain or bleeding lasts longer than a week, medical evaluation may be needed to confirm the cause and rule out other conditions.

3. A Peptic Ulcer

A peptic ulcer develops when an open sore forms in the lining of the stomach or in the first part of the small intestine, called the duodenum. This happens when stomach acid damages the protective layer that normally shields the digestive tract. You may notice a dull or burning pain in the stomach, often occurring between meals or at night.

Eating food or taking acid-reducing medicine may ease the pain temporarily, but it can return later. In some cases, ulcers bleed, which can cause blood in the stool or black, tar-like stools. The most common causes of peptic ulcers are infection with Helicobacter pylori bacteria and long-term use of pain relievers such as nonsteroidal anti-inflammatory drugs.

Stress or spicy foods may worsen symptoms but do not directly cause ulcers. To diagnose a peptic ulcer, tests such as an endoscopy, breath test, or stool test may be used.

4. Diverticulitis

Diverticulitis occurs when small pouches in the colon, called diverticula, become inflamed or infected. These pouches develop over time as pressure builds inside the colon, often linked to low-fiber diets. When a pouch becomes irritated or blocked, it can trigger pain, bleeding, and bowel changes.

Common symptoms include lower abdominal pain, usually on the left side, along with fever, nausea, vomiting, bloating, and changes in bowel habits such as constipation or diarrhea. Fatigue may also occur as the body responds to infection or inflammation. You may notice blood in your stool, which can appear bright red or dark.

Bleeding may start suddenly without pain or follow abdominal cramping. In many cases, the bleeding stops on its own, but heavy or ongoing bleeding needs medical attention. In more serious cases, diverticulitis can lead to complications such as abscesses, perforation of the colon, fistulas, or bowel blockages.

5. Gastroenteritis

Gastroenteritis occurs when the stomach and intestines become irritated or infected, most often from food or water contaminated with viruses, bacteria, or parasites. This infection inflames the digestive tract and disrupts normal digestion, with symptoms usually beginning within a day or two of exposure.

You may notice sudden diarrhea that can be watery or contain small amounts of blood or mucus. Other common symptoms include nausea, vomiting, stomach cramps, fever, and fatigue. When blood appears in the stool, it often means the intestinal lining is inflamed or damaged. Bacterial infections, such as certain strains of E. coli or Shigella, are more likely to cause bleeding than viral infections.

Ongoing diarrhea and vomiting can lead to dehydration due to fluid loss. Most cases improve with rest, drinking plenty of fluids, and eating bland foods. You can reduce your risk by washing your hands often, cooking food thoroughly, and avoiding unsafe water sources.

6. Irritable Bowel Syndrome

Irritable bowel syndrome, or IBS, affects how your digestive system works. It does not cause visible damage in the intestines but can still lead to uncomfortable symptoms. Common signs include cramping, bloating, gas, diarrhea, constipation, or both. These symptoms often change over time and can be mild or severe.

You may notice that IBS symptoms come and go. Flare-ups can last for days or weeks before easing again. Certain triggers, including specific foods, stress, or hormonal changes, often make symptoms worse. Although IBS does not usually cause blood in the stool, it can make you more aware of changes in bowel habits, which helps identify when another condition may be present.

Doctors typically diagnose IBS after ruling out other causes with similar symptoms, such as infections or inflammatory bowel disease. Tests like blood work, stool studies, or imaging help confirm that there is no inflammation, bleeding source, or structural problem.

7. Ulcerative Colitis

Ulcerative colitis is a long-term condition that causes inflammation and sores in the lining of your colon and rectum. These open sores, called ulcers, can bleed and lead to blood appearing in your stool. The amount of bleeding may vary depending on how much of the colon is affected.

Common symptoms often develop along with bleeding. You might notice mucus in your stool, an urgent need to use the bathroom, ongoing diarrhea, stomach cramps, and fatigue. Symptoms tend to come and go, with periods of remission followed by flare-ups that can last days or weeks.

The condition occurs when the immune system reacts abnormally and attacks the digestive tract. Although the exact cause is not fully understood, factors such as genetics, gut bacteria, and environmental influences may contribute. Stress and certain foods can worsen symptoms but do not directly cause ulcerative colitis.

8. Colorectal Cancer

Colorectal cancer begins in the colon or rectum, which are parts of the large intestine responsible for processing waste. It often starts as small growths called polyps. These growths are usually harmless at first but can slowly turn into cancer if they are not found and removed early.

In the early stages, you may not notice any symptoms. As the disease progresses, signs can appear, including blood in the stool, dark or tar-like stools, or ongoing changes in bowel habits such as constipation or diarrhea. Some people also experience stomach pain, cramping, fatigue that does not improve, or unexplained weight loss.

Colorectal cancer can cause bleeding inside the digestive tract. This blood may mix with the stool or show up as streaks on toilet paper. Doctors rely on screening tests, such as a colonoscopy, to examine the colon and detect polyps or early-stage cancer.

9. Angiodysplasia

Angiodysplasia occurs when tiny blood vessels in the colon or small intestine become weak and enlarged. These fragile vessels can bleed easily, which may lead to blood appearing in the stool. The condition most often affects older adults because blood vessels can weaken with age.

Bleeding from angiodysplasia is often painless. Some people see bright red blood or dark, tar-like stools, while others have no visible symptoms and find out through stool tests. When bleeding happens repeatedly, even in small amounts, it can lead to anemia. This may cause fatigue, weakness, or shortness of breath over time.

Doctors diagnose angiodysplasia using tests such as a colonoscopy, CT angiography, or blood tests that check red blood cell levels. In some cases, the bleeding stops on its own and does not require treatment. If bleeding continues, treatment may be needed. During a colonoscopy, doctors can seal the affected blood vessels using heat or laser.

10. Gastrointestinal (GI) Bleeding

Gastrointestinal bleeding occurs when blood comes from any part of the digestive system, including the esophagus, stomach, small intestine, or large intestine. The bleeding may be obvious or so small that only a laboratory test can detect it. You might notice blood in your stool or vomit, or your stool may appear dark and tar-like.

The color and appearance of the blood can help suggest where the bleeding is coming from. Bright red blood often points to bleeding in the lower intestines or rectum. Dark maroon or black stools usually indicate bleeding higher in the digestive tract, such as in the stomach or upper intestine. Common causes include ulcers, hemorrhoids, small tears in the esophagus, or inflammation in the stomach or intestines.

Long-term conditions such as Crohn’s disease, ulcerative colitis, or colon polyps can also cause bleeding. In more serious cases, cancers of the digestive tract may be responsible.

11. Polyps

Polyps are small growths that form on the inner lining of the colon or rectum. They can be round, flat, or attached by a thin stalk. Most polyps are harmless, but some can slowly develop into cancer if they are not removed over time.

Many people with polyps do not have any symptoms. When symptoms do occur, they may include blood in the stool, on toilet paper, or in the toilet bowl after a bowel movement. Some people also notice changes in bowel habits, such as diarrhea, constipation, or mucus in the stool that lasts longer than a week.

Certain factors increase the risk of developing polyps. These include being over age 50, having a personal or family history of colon polyps or colon cancer, or having had polyps in the past. In rare cases, inherited conditions can cause multiple polyps to form at a younger age. Polyps are often found during routine screening tests such as a colonoscopy.

During this procedure, doctors usually remove the polyps in a process called a polypectomy. The removed tissue is then examined in a laboratory to determine the type of polyp and check for abnormal or precancerous cells.

12. Radiation Damage

Radiation damage occurs when exposure harms the lining of the digestive tract. This can happen after cancer treatments such as radiation therapy or from exposure to high levels of ionizing radiation. Damage to this lining may lead to bleeding in the stool.

When the intestines or colon receive a large dose of radiation in a short period, the cells that line these organs can become inflamed or begin to die. This condition, sometimes called radiation colitis or proctitis, may cause stomach pain, diarrhea, and stools that appear dark or contain blood. The risk is higher when radiation is directed near the abdomen or pelvis.

Over time, repeated or high-dose radiation can weaken small blood vessels in the intestinal wall. These fragile vessels may rupture and bleed, leading to small amounts of blood mixed with stool or seen on toilet paper.

13. Perianal Abscess

A perianal abscess is a pocket of pus that forms near the anus when small glands in the anal area become blocked and infected by bacteria. This infection leads to inflammation, swelling, and pain, often creating a tender lump near the opening of the anus.

You may notice redness, warmth, or throbbing pain around the area. The discomfort often worsens when sitting, coughing, or passing stool. In some cases, fever, fatigue, or a general feeling of illness can develop if the infection spreads beyond the local area.

Several factors can increase the risk of developing a perianal abscess. These include inflammatory bowel disease, diabetes, or conditions that weaken the immune system. Anal trauma, small cuts, or irritation can also allow bacteria to enter and trigger infection. Antibiotics are typically used only if the infection spreads or if immune defenses are reduced. Warm sitz baths may help ease discomfort and support recovery.

14. Fistula

A fistula is an abnormal tunnel that forms between two parts of the body that are not normally connected. In the digestive system, it can link sections of the intestine to other organs or to the skin. When this happens, waste or fluids may pass through an unusual opening, and blood can sometimes appear in the stool.

Fistulas often develop because of ongoing inflammation, infection, surgery, or injury. Conditions that cause long-term irritation in the intestines make fistulas more likely to form. For example, chronic inflammation can weaken tissue and create openings between nearby structures.

If you have a bowel fistula, you may notice symptoms such as abdominal pain, drainage from the skin, or bloody stool mixed with pus. The color and texture of the fluid leaking from a fistula can help doctors find its source. For example, a brown, stool-like discharge may come from the large intestine.