Bright red blood in stool most often comes from the lower part of the digestive tract, typically the rectum or colon. The most common cause in adults is hemorrhoids, but anal fissures, diverticular bleeding, infections, and inflammatory bowel disease can all produce visible red blood. The color matters: bright red generally means the bleeding source is close to the exit, while dark or tarry stool suggests bleeding higher up in the digestive system.
Hemorrhoids
Hemorrhoids are the most common cause of rectal bleeding in middle-aged and older adults. They’re cushions of tissue and blood vessels in the anal canal that become symptomatic when the supporting muscle fibers weaken. Internal hemorrhoids cause painless bleeding, often noticed as bright red streaks on toilet paper or drops in the toilet bowl. You might bleed during a bowel movement without feeling any discomfort at all.
External hemorrhoids, on the other hand, typically don’t bleed unless they become thrombosed (when a blood clot forms inside them). At that point, they cause significant pain and swelling. The key distinguishing feature of hemorrhoid bleeding is that it’s usually painless, intermittent, and triggered by straining.
Anal Fissures
An anal fissure is a small tear in the lining of the anus, most often caused by passing hard stools during constipation. Childbirth can also cause fissures, particularly along the front of the anal canal. Unlike hemorrhoids, fissures produce sharp, burning pain that can last for hours after a bowel movement. The bleeding is typically small in volume: a streak of bright red blood on the stool or on toilet paper.
If you’re trying to distinguish between the two, pain is the clearest signal. Hemorrhoids tend to cause a dull ache or itch that comes and goes. Fissures produce intense, sharp pain concentrated during and after passing stool. Both can cause blood on toilet paper, but hemorrhoids tend to bleed more noticeably, sometimes with larger clots.
Diverticular Bleeding
Diverticula are small pouches that form in the wall of the colon, usually at weak points where blood vessels pass through the muscle layer. They’re extremely common with age and often cause no symptoms at all. But in roughly 5% to 15% of people who have them, the blood vessels within these pouches become fragile over time. As the vessel wall thins, it can rupture and bleed directly into the colon.
Diverticular bleeding is painless, which can make it alarming. It often comes on suddenly and can be heavy, producing a large amount of bright red or maroon-colored blood. Most episodes stop on their own, but significant bleeding from diverticular disease is one of the more common reasons people end up in the emergency room for rectal bleeding.
Inflammatory Bowel Disease
Ulcerative colitis and Crohn’s disease both involve chronic inflammation of the digestive tract and can cause blood in the stool. Ulcerative colitis is limited to the colon and rectum, while Crohn’s disease can affect any part of the digestive system and tends to penetrate deeper into the bowel wall.
With ulcerative colitis, rectal bleeding is often one of the earliest and most prominent symptoms. You might notice blood mixed with diarrhea, along with mucus or pus. Other hallmarks include an urgent need to pass stool, cramping abdominal pain, and the frustrating sensation of needing to go but being unable to. In mild cases where only the rectum is inflamed, bleeding or urgency may be the only signs. Diagnosis requires a colonoscopy with tissue biopsy. Stool tests can also detect markers of inflammation and rule out infections.
Bacterial Infections
Several types of food-borne bacteria can cause bloody diarrhea, which may look like bright red blood in stool. The timeline and pattern vary by pathogen. A common type of E. coli starts with painful, watery diarrhea that becomes bloody after one to three days. Campylobacter infections can cause up to 10 bowel movements a day, with bloody stools appearing on the second or third day in about 15% of adult patients (and closer to 50% of children). Shigella infections often produce blood and mucus in the stool along with a painful urge to defecate.
Bacterial infections usually come with additional symptoms that set them apart from other causes: fever, abdominal cramps, and a clear onset after eating contaminated food or traveling. These infections typically resolve within days to a couple of weeks, though some require treatment.
Medications That Increase Bleeding Risk
Regular use of common pain relievers like ibuprofen and naproxen can damage the lining of the digestive tract from top to bottom, including the colon. Mucosal injury, including erosions and small ulcers, has been observed in up to 70% of people taking these medications in studies using capsule endoscopy. This damage can cause bleeding that ranges from invisible (leading to iron deficiency over time) to overtly bloody stool.
The risk increases substantially when these pain relievers are combined with blood thinners. People taking dual antiplatelet therapy along with one of these pain relievers have roughly twice the risk of gastrointestinal bleeding compared to those not taking the pain reliever. If you’re on blood thinners or take over-the-counter pain relievers regularly and notice blood in your stool, that combination is worth mentioning to your doctor.
Colorectal Cancer
While far less common than hemorrhoids or fissures, colorectal cancer can cause bright red blood in stool, particularly when a tumor is located in the rectum or lower colon. The bleeding may be intermittent and easy to dismiss. Other signs include a persistent change in bowel habits, unexplained weight loss, or a feeling that the bowel doesn’t empty completely.
The U.S. Preventive Services Task Force recommends that all adults begin screening for colorectal cancer at age 45 and continue through age 75. Screening is designed for people without symptoms. If you have symptoms like rectal bleeding, that’s a reason for diagnostic testing rather than routine screening, regardless of your age.
Signs That Need Immediate Attention
Most causes of bright red blood in stool are not emergencies, but certain combinations of symptoms signal serious bleeding that requires immediate care. Heavy or continuous rectal bleeding, especially with severe abdominal pain or cramping, warrants a trip to the emergency room.
Call emergency services if rectal bleeding is accompanied by signs of significant blood loss:
- Dizziness or lightheadedness when standing
- Rapid, shallow breathing
- Cold, clammy, or pale skin
- Confusion or fainting
- Blurred vision
- Very low urine output
Reducing Straining-Related Bleeding
Since hemorrhoids and anal fissures account for the majority of bright red rectal bleeding, preventing straining during bowel movements goes a long way. The most effective strategy is increasing dietary fiber. Current guidelines recommend 14 grams of fiber per 1,000 calories consumed, which works out to about 28 grams per day on a standard 2,000-calorie diet. Good sources include beans, whole grains, fruits, and vegetables.
Fiber works best when paired with adequate fluid intake, since fiber without water can actually worsen constipation. Drinking water, fruit juices, and clear soups throughout the day helps keep stool soft and easier to pass. Building fiber intake gradually over a week or two also helps avoid the bloating and gas that can come from a sudden increase.