Bloody diarrhea has several possible causes, ranging from a short-lived bacterial infection to a chronic condition like inflammatory bowel disease. The color and amount of blood, whether you have a fever, and how long symptoms have lasted all help narrow down what’s going on. If you’re passing large amounts of blood, feel lightheaded, notice a rapid heart rate, or feel unusually weak, that’s a medical emergency.
What the Color of Blood Tells You
Bright red or maroon blood in your stool typically means the bleeding is coming from your large intestine or rectum. The blood is fresh and hasn’t traveled far, so it keeps its red color. This is the pattern most people with bloody diarrhea notice.
Black, tarry, sticky stool is a different signal. It means blood has traveled from much higher up, usually the stomach or upper small intestine. Digestive chemicals break down the blood along the way, turning it dark. A small amount of upper GI bleeding can look dark brown rather than jet black. If your stool looks like this, the cause is different from most bloody diarrhea and needs prompt evaluation.
Bacterial Infections
A bacterial infection is one of the most common reasons for sudden bloody diarrhea, especially if it started within the last few days. The usual culprits are E. coli O157:H7, Shigella, Salmonella, and Campylobacter. You typically pick these up from contaminated food or water.
The pattern of symptoms can hint at which bacterium is involved. A high fever (above about 102°F) raises suspicion for Shigella. E. coli O157:H7, on the other hand, rarely causes fever by the time you see a doctor, even though about half of people infected with it recall having a fever earlier on. Both infections can cause significant cramping along with the bloody stool. Most bacterial cases resolve on their own, though E. coli O157:H7 requires close monitoring because it can, in rare cases, lead to kidney complications.
Ulcerative Colitis and Inflammatory Bowel Disease
If bloody diarrhea keeps coming back or has lasted weeks, ulcerative colitis is a strong possibility. This is a type of inflammatory bowel disease where the immune system attacks the innermost lining of the colon and rectum, creating chronic inflammation and small ulcers. Those ulcers bleed, and the inflammation drives frequent, loose stools, sometimes mixed with mucus.
The exact cause isn’t fully understood, but it appears to involve an abnormal immune response, possibly triggered by gut bacteria, that also damages your own tissue. Symptoms tend to flare and then improve in cycles. During a flare, you might have urgency, cramping, and dozens of bowel movements a day.
Diagnosis usually involves blood tests to check for anemia and signs of inflammation, stool tests that look for specific proteins associated with the condition, and ultimately a colonoscopy with a tissue biopsy. The biopsy is the only definitive way to confirm ulcerative colitis.
Pain Medications and Other Drugs
Over-the-counter painkillers like ibuprofen and naproxen are a frequently overlooked cause of GI bleeding. These drugs weaken the protective lining of your digestive tract in multiple ways: they reduce the blood flow to the stomach and intestinal walls, compromise the waterproof barrier that normally shields the tissue from stomach acid and bile, and can directly damage cells. The result is erosions or ulcers that bleed.
If you’ve been taking these painkillers regularly, especially on an empty stomach or alongside alcohol, they could be the source. Blood thinners and even high-dose aspirin can also contribute. The bleeding from these medications often shows up as dark or black stool rather than bright red, but it can look either way depending on where the damage occurs.
Ischemic Colitis
Ischemic colitis happens when blood flow to part of the colon drops, starving the tissue of oxygen. It causes sudden cramping (usually on the left side), an urgent need to go, and bright red or maroon blood in the stool. The condition mostly affects adults over 60, particularly those with atherosclerosis, heart failure, diabetes, or low blood pressure.
Less obvious triggers include dehydration, previous abdominal surgery (scar tissue can restrict blood flow), and even intense endurance exercise like marathon running. Cocaine and methamphetamine use are also well-known causes in younger people. Most mild cases heal on their own within a few days, but severe episodes where the tissue dies require hospital treatment.
Hemorrhoids and Diverticular Bleeding
These two causes produce blood in stool but not always diarrhea. Still, they’re worth knowing about because they’re common and can overlap with loose stools from other causes.
- Hemorrhoids cause bleeding that’s typically noticed on toilet paper or in the bowl, associated with bowel movements. They’re usually painless unless a blood clot forms inside one, and itching is common.
- Diverticular bleeding is sudden, painless, and can be heavy. It happens when small pouches in the colon wall (diverticula) erode into a blood vessel. There’s no cramping beforehand, which distinguishes it from most other causes of bloody diarrhea.
Colorectal Cancer Warning Signs
Colorectal cancer is the concern many people jump to, and while it’s less common than the causes above, it’s worth taking seriously. It’s the third most common cancer in the United States among both men and women, and new cases in people under 50 have been rising since the mid-1990s. Half of early-onset colorectal cancers are now diagnosed in people under 45.
A National Cancer Institute study found that four warning signs appeared more often in the three months to two years before a colorectal cancer diagnosis: abdominal pain, rectal bleeding, diarrhea, and iron deficiency anemia. Having just one of these signs was associated with nearly twice the likelihood of an early-onset diagnosis compared to having none. Having three or more was associated with six times the likelihood. Nearly 20% of younger adults with colorectal cancer had at least one of these signs well before diagnosis.
This doesn’t mean bloody diarrhea equals cancer. It means persistent or recurring symptoms, especially combinations of them, deserve a proper evaluation rather than a wait-and-see approach.
What Happens During a Workup
When you seek care for bloody diarrhea, the evaluation usually starts with your history: how long symptoms have lasted, what the blood looks like, whether you have fever or weight loss, what medications you take, and whether you’ve traveled recently or eaten anything questionable. From there, common next steps include stool tests to check for bacterial infections and inflammatory markers, blood tests to assess for anemia and infection, and potentially imaging or a colonoscopy if the cause isn’t immediately clear. The sequence depends on whether your symptoms suggest something acute, like food poisoning, or something chronic, like inflammatory bowel disease or cancer.