Rectal bleeding has a wide range of causes, from hemorrhoids and small tears in the skin to serious conditions like colorectal cancer. The most common culprits are hemorrhoids, anal fissures, and constipation, but the color, amount, and pattern of bleeding all offer clues about where it’s coming from and how urgent it is.
What the Color of Blood Tells You
The color of blood you see is one of the most useful clues to the source. Bright red blood on toilet paper or in the bowl typically points to a problem in or near the anus or lower colon, such as hemorrhoids, a fissure, or a polyp. The blood is red because it hasn’t traveled far or been exposed to digestive enzymes.
Dark red or maroon blood mixed into the stool suggests bleeding higher up in the colon, potentially from diverticular disease, inflammatory bowel disease, or a growth. Black, tarry, foul-smelling stools (called melena) signal bleeding even further upstream, usually above the colon entirely. It takes roughly 100 to 200 cc of blood in the upper digestive tract to produce that black, sticky appearance, because digestive enzymes chemically alter the blood as it passes through.
Hemorrhoids
Hemorrhoids are swollen veins in the anus or rectum, and they’re by far the most common reason people notice blood after a bowel movement. The bleeding is usually bright red and shows up on toilet paper, on the surface of the stool, or dripping into the bowl. Hemorrhoids can produce a surprisingly noticeable amount of blood, sometimes including small clots, which can be alarming even though the condition itself is rarely dangerous.
Pain from hemorrhoids tends to be a dull ache or itch that comes and goes. Internal hemorrhoids often cause painless bleeding, while external ones are more likely to be uncomfortable, especially if a clot forms inside them. Straining during bowel movements, sitting for long periods, pregnancy, and chronic constipation all contribute to their development.
Anal Fissures
An anal fissure is a small tear in the lining of the anal canal, usually caused by passing a hard or large stool. The key difference from hemorrhoids is the pain: fissures produce a sharp, burning sensation that can last for hours after a bowel movement. If your pain feels like a tear during or right after going to the bathroom, a fissure is the more likely explanation.
Fissures produce smaller amounts of bright red blood compared to hemorrhoids. Most heal on their own within a few weeks with softer stools and adequate hydration, though chronic fissures sometimes need additional treatment.
Constipation and Hard Stools
Chronic constipation can cause rectal bleeding even without a diagnosable fissure or hemorrhoid. Hard stools scrape and irritate the lining of the rectum and anus on their way out, producing streaks of bright red blood. This is especially common in people who strain regularly. Constipation also worsens existing hemorrhoids and fissures, creating a cycle where the bleeding keeps returning until the underlying stool consistency improves.
Diverticular Bleeding
Diverticulosis refers to small pouches that form in the wall of the colon, and it becomes increasingly common with age. About 5% of people have diverticula by age 40, rising to roughly 65% of people by age 85 in Western countries. Most people with diverticula never have symptoms, but 3% to 5% of them develop bleeding.
When diverticular bleeding happens, it’s distinctive: painless and often heavy, producing large amounts of dark red or maroon blood in the stool. Episodes tend to start suddenly and stop on their own, but the volume of blood can be significant enough to require medical evaluation. This is one of the most common causes of major lower gastrointestinal bleeding in older adults.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease both cause chronic inflammation in the digestive tract, and rectal bleeding is a hallmark symptom. In ulcerative colitis, the immune system attacks the lining of the colon and rectum, creating ulcers that bleed. The result is often diarrhea mixed with blood, mucus, or pus, along with urgency and cramping.
When inflammation is limited to the rectum (a form called ulcerative proctitis), rectal bleeding or urgency may be the only noticeable sign. Flare-ups can produce frequent bloody stools for weeks, followed by periods of remission. Crohn’s disease can affect any part of the digestive tract and produces similar bleeding when the colon or rectum is involved.
Polyps and Colorectal Cancer
Colon polyps are abnormal growths on the inner lining of the colon. Most are harmless, but some can develop into cancer over time. Both polyps and colorectal cancer can cause small amounts of blood in the stool, often invisible to the naked eye and detectable only through screening tests. When bleeding is visible, it may appear as dark red blood mixed into the stool rather than bright red blood on the surface.
Colorectal cancer has traditionally been considered a disease of older adults, but early-onset cases are rising sharply. A large international analysis published in The Lancet Oncology found that colorectal cancer rates in adults under 50 are increasing in 27 of 50 countries studied, with some of the steepest annual increases occurring in New Zealand, Chile, and England. This trend is why screening recommendations have shifted to begin at age 45 in many guidelines. Rectal bleeding accompanied by unexplained weight loss, changes in bowel habits lasting more than a few weeks, or a feeling that the bowel doesn’t fully empty warrants prompt evaluation regardless of age.
Medications That Increase Bleeding Risk
Certain medications don’t cause bleeding on their own but significantly raise the risk by interfering with blood clotting or damaging the digestive lining. Common anti-inflammatory painkillers (like ibuprofen and naproxen) roughly quadruple the risk of upper gastrointestinal bleeding compared to not taking them. Low-dose aspirin triples the risk.
The danger multiplies when these drugs are combined. Taking an anti-inflammatory painkiller alongside a corticosteroid increases the bleeding risk nearly 13-fold. Combining these painkillers with blood thinners or with certain antidepressants (SSRIs) also produces excess risk beyond what either drug would cause alone. If you take any combination of these medications and notice blood in your stool, that context is important information for your doctor.
Less Common Causes
Several other conditions can produce rectal bleeding, though they account for a smaller share of cases. Angiodysplasia involves fragile, abnormal blood vessels in the intestinal wall that can leak blood, particularly in older adults. Infections from bacteria, viruses, or parasites can inflame the colon and cause bloody diarrhea. Radiation therapy to the pelvic area can damage the rectal lining and cause bleeding that sometimes appears months or years after treatment. Anal cancer is rare but can present with bleeding that mimics hemorrhoids.
How Rectal Bleeding Is Evaluated
The evaluation typically starts with a physical exam and a digital rectal exam, where a provider feels for hemorrhoids, fissures, or other abnormalities. For bleeding that appears to come from the anus or lower rectum, an anoscopy is a common next step. This involves inserting a short, lighted tube about two inches into the anus to visually inspect the area. It can identify hemorrhoids, fissures, polyps, and signs of inflammation. If anything looks abnormal, a small tissue sample can be taken during the same procedure.
When the source of bleeding isn’t obvious or when there are risk factors for more serious conditions, a colonoscopy examines the entire colon. Stool tests that detect hidden blood or cancer-related DNA markers may also be used as initial screening tools.
Signs That Need Urgent Attention
Most rectal bleeding is minor and resolves on its own or with simple treatment. But certain patterns require immediate care. Heavy or continuous bleeding that doesn’t stop, or bleeding paired with severe abdominal pain, warrants a trip to the emergency room. Signs that significant blood loss is affecting your body include rapid shallow breathing, dizziness when standing, blurred vision, fainting, confusion, nausea, cold or clammy skin, and reduced urine output. These indicate your circulatory system is under stress and you need emergency help.