The most common reason you see blood in your stool during constipation is a small tear in the lining of your anus, called an anal fissure, caused by passing hard or unusually large stool. Hemorrhoids, which are swollen blood vessels around the rectum, run a close second. Both conditions are directly provoked by the straining and hard stools that define constipation, which is why the two problems so often travel together. The blood is usually bright red and shows up on the toilet paper or on the surface of the stool rather than mixed into it, and while it can look alarming, the cause is benign in the vast majority of cases.
Anal Fissures and Why Constipation Creates Them
An anal fissure is a small longitudinal tear in the thin, sensitive tissue that lines the anal canal.1Medicine Science | International Medical Journal. Conservative management of anal fissure accompanying constipation in school-age children When stool is hard and dry, as it typically is during constipation, it stretches the anal opening beyond what that tissue can handle. The result is a tear that bleeds, stings, and often burns during and after a bowel movement. If you have ever noticed bright red blood on the toilet paper accompanied by a sharp, stinging pain right at the opening, a fissure is the most likely explanation.
What makes fissures frustrating is the cycle they create. The tear hurts, so you instinctively tighten the sphincter muscles, which reduces blood flow to the area and slows healing. Meanwhile, the pain may make you dread going to the bathroom, which leads to holding stool longer, which makes it harder, which makes the next tear worse. Breaking that cycle is the entire strategy behind treatment, which we will get to later in the article.
Fissures are not limited to adults. They are one of the most common causes of rectal bleeding in children, where constipation is especially prevalent. A child who suddenly refuses to sit on the toilet or cries during a bowel movement may be dealing with a fissure rather than behavioral stubbornness.
Hemorrhoids and Straining
Hemorrhoids are cushions of blood vessels that everyone has in and around the rectum. They become a problem when they swell, stretch, or become irritated, typically from repeated straining. Constipation is probably the single biggest driver of symptomatic hemorrhoids, because every episode of pushing against hard stool increases pressure in those vessels.
Internal hemorrhoids sit inside the rectum where you cannot feel them and tend to bleed painlessly, leaving bright red blood in the bowl or on the paper. External hemorrhoids sit under the skin around the anus and can itch, swell, or become tender. Occasionally an external hemorrhoid develops a blood clot (thrombosis), which causes a sudden, firm, painful lump at the anal margin. This can bleed if the overlying skin breaks open.
The blood from hemorrhoids is usually bright red, similar to fissure bleeding. The key difference is that hemorrhoids tend to produce painless bleeding (unless thrombosed), while fissures almost always hurt during the bowel movement itself. Both conditions respond well to the same first-line approach: soften the stool, stop straining, and let the tissue recover.
What the Blood Looks Like Matters
Not all rectal bleeding is the same, and paying attention to the color and pattern can help you and your doctor sort out what is going on. Bright red blood, especially when it coats the outside of the stool or shows up only on the tissue, almost always comes from a source near the anal opening: a fissure, a hemorrhoid, or a minor abrasion. This is the type most closely linked to constipation.
Dark red or maroon blood mixed into the stool suggests the bleeding source is higher up in the colon. Black, tarry stools with a distinctive foul smell (called melena) indicate bleeding from the stomach or upper intestine, where the blood has been partially digested. Neither of these patterns is typical of constipation-related bleeding, and both warrant prompt medical attention.
A common misconception is that any amount of blood in the stool means something serious. For otherwise healthy people dealing with a bout of constipation, a streak of bright red blood that resolves once stool softens is rarely dangerous. That said, ongoing bleeding, bleeding that changes in character, or bleeding accompanied by other symptoms like unexplained weight loss, fatigue, or a change in bowel habits that persists for weeks should always be evaluated.
Less Common Causes Worth Knowing About
While fissures and hemorrhoids explain the overwhelming majority of constipation-related rectal bleeding, several other conditions can produce blood in the stool and deserve mention because missing them has real consequences.
- Solitary rectal ulcer syndrome: Chronic straining can damage the rectal wall itself, producing ulcers that bleed. This condition is associated with pelvic floor dysfunction, where the muscles involved in defecation do not coordinate properly (a pattern called dyssynergia).2PubMed Central. Treating pelvic floor disorders of defecation: management or cure? People with solitary rectal ulcer syndrome often report mucus discharge alongside blood and a persistent feeling of incomplete evacuation.
- Ischemic colitis: Reduced blood flow to the colon can cause inflammation, pain, and bloody diarrhea or bloody stool. Most cases occur in older adults with cardiovascular disease, but the condition can appear in younger patients with different risk factors, which makes it easy to overlook.3PubMed Central. Ischemic colitis Ischemic colitis is not caused by constipation itself, but severe constipation with fecal impaction can contribute to vascular compromise in the colon wall.
- Inflammatory bowel disease: Crohn’s disease and ulcerative colitis produce chronic inflammation in the gut that leads to bloody stools, diarrhea, and abdominal pain. These conditions are not caused by constipation, but constipation can coexist with them, especially in Crohn’s disease when inflammation narrows the bowel.
- Colorectal polyps or cancer: Polyps in the colon can bleed intermittently, and colorectal cancer is a rarer but critical cause of rectal bleeding. The blood from a polyp or tumor may be dark, intermittent, and mixed into the stool rather than sitting on its surface. Anyone over 45 with new rectal bleeding, or younger adults with a family history of colorectal cancer, should discuss screening with their doctor.
The takeaway from that list is not to panic, but to pay attention to context. If you are constipated, pass a hard stool, and see a small amount of bright red blood that goes away, you can be fairly confident the cause is mechanical. If blood keeps appearing, changes character, or comes with other symptoms, the investigation needs to go deeper.
Bleeding During Pregnancy and Postpartum
Pregnant women experience constipation at high rates, thanks to hormonal shifts that slow gut motility and the growing uterus pressing on the rectum. Rectal bleeding during pregnancy is common and, in most cases, comes from hemorrhoids or anal fissures, the same benign causes that affect everyone else.4PubMed Central. Lower gastrointestinal bleeding in pregnancy: Differential diagnosis, assessment and management However, more serious diagnoses like inflammatory bowel disease flares or, rarely, colorectal malignancy should not be dismissed simply because the patient is pregnant.4PubMed Central. Lower gastrointestinal bleeding in pregnancy: Differential diagnosis, assessment and management
Iron supplements, which are prescribed to nearly every pregnant woman, add an extra layer of confusion. Iron is notorious for turning stools black and causing constipation, which can make it hard to tell whether dark stool is a medication effect or a sign of upper gastrointestinal bleeding. If you are pregnant, taking iron, and notice black stools, mention it to your provider but know that the supplement is the most likely culprit. Bright red bleeding on the paper is a separate issue and is usually from hemorrhoids or a fissure aggravated by pregnancy-related constipation.
Postpartum bleeding from the rectum also catches many new parents off guard. The combination of weakened pelvic floor muscles, residual hemorrhoids from labor, and the constipation that follows delivery (especially after opioid pain medication) sets the stage for fissures and hemorrhoid flares in the weeks after giving birth.
When Pelvic Floor Dysfunction Keeps the Cycle Going
Some people have chronic constipation that does not respond to extra fiber and water because the underlying problem is muscular rather than dietary. Dyssynergic defecation is a condition where the pelvic floor muscles contract instead of relaxing during a bowel movement, essentially working against the effort to push stool out. This leads to excessive straining, incomplete evacuation, and repeated trauma to the anal canal, all of which increase the risk of bleeding from fissures and hemorrhoids.
Dyssynergia can coexist with structural problems like solitary rectal ulcer syndrome or rectocele (a bulging of the rectal wall into the vagina).2PubMed Central. Treating pelvic floor disorders of defecation: management or cure? If you find yourself straining hard at every bowel movement despite soft stool, or if you feel like stool gets “stuck” and you can never fully empty, pelvic floor dysfunction is worth investigating. A specialized test called anorectal manometry can measure how your muscles behave during defecation.
Treatment for dyssynergic defecation centers on biofeedback therapy, where a therapist uses sensors to show you your pelvic floor muscle activity in real time and teaches you to relax those muscles at the right moment. Research has found that combining biofeedback with targeted pelvic floor and abdominal exercises improves patient outcomes more than either approach alone.5PubMed. Efficacy of combination of biofeedback therapy and pelvic floor muscle training in dyssynergic defecation This is not a quick fix, but for people whose constipation and bleeding have resisted standard dietary changes, it can break the cycle that fiber supplements never could.
How to Stop the Bleeding by Fixing the Constipation
The single most effective thing you can do about constipation-related rectal bleeding is prevent the constipation. Once stool is soft enough to pass without straining, fissures heal and hemorrhoids shrink on their own in the vast majority of cases. The approach is straightforward but requires consistency.
Fiber intake is the foundation. Most adults eat far less fiber than the recommended range, and the shortfall directly contributes to hard stools. Increasing fiber gradually (not all at once, which causes bloating and gas) through fruits, vegetables, legumes, and whole grains makes stool bulkier and softer. A fiber supplement like psyllium husk works well if dietary changes are not enough. Adequate water intake matters alongside fiber: fiber without water can actually worsen constipation.
Physical activity helps too. Regular movement stimulates the rhythmic contractions of the colon that propel stool forward. Even a daily walk can make a measurable difference for people with sluggish bowels. Avoiding the habit of delaying bowel movements is equally important. When you ignore the urge, the colon continues absorbing water from the stool, making it harder and more painful to pass later.
For fissures that have become chronic (lasting more than six weeks), over-the-counter measures like stool softeners and sitz baths are helpful but sometimes not sufficient. Prescription topical treatments that relax the internal anal sphincter can promote healing by restoring blood flow to the tear. In a study of patients treated with topical diltiazem ointment, about three-quarters experienced healing within two to three months, and most of the remaining patients healed with an additional eight weeks of treatment.6PubMed. Topical diltiazem ointment in the treatment of chronic anal fissure Nitroglycerin ointment works through a similar mechanism. Surgery (lateral internal sphincterotomy) is reserved for fissures that fail medical therapy and is highly effective, though it carries a small risk of affecting continence.
When to See a Doctor
A one-off episode of bright red blood during a hard bowel movement, in an otherwise healthy person, is common enough that it rarely requires an urgent visit. But there are situations where you should not wait:
- Heavy bleeding: If blood is dripping into the bowl, soaking through clothing, or continuing after the bowel movement ends, that volume goes beyond a typical fissure or hemorrhoid.
- Dark or black stools: This suggests a bleeding source higher in the digestive tract and needs investigation, unless you are taking iron supplements or bismuth-containing medications (like Pepto-Bismol), which also darken stool.
- Persistent bleeding: Blood that keeps showing up over several weeks, even if small in amount, deserves evaluation.
- Accompanying symptoms: Unexplained weight loss, new fatigue, abdominal pain that does not go away, or a sustained change in your bowel pattern (like new alternating constipation and diarrhea) are reasons to get checked.
- Age or family history: If you are over 45 and have not had a colonoscopy, new rectal bleeding is a reasonable prompt to schedule one. If you have a first-degree relative who had colorectal cancer, screening often starts earlier.
Doctors typically start with a physical examination that includes a visual inspection of the anus and a digital rectal exam. For persistent symptoms, an anoscopy (a quick look inside the anal canal with a small scope) or a colonoscopy may follow. These are not as uncomfortable as people fear, and they provide the clearest answers.
Medications That Muddy the Picture
Several common medications can either cause constipation, change stool color, or irritate the gut lining, all of which complicate the question of why you are seeing blood.
Opioid pain medications are among the strongest constipation-causing drugs. They slow gut motility dramatically, producing hard, infrequent stools that are especially likely to cause fissures. If you are taking opioids after surgery or for chronic pain, a stool softener or osmotic laxative taken preventively (not just when constipation sets in) is the standard recommendation.
Blood thinners like warfarin or direct oral anticoagulants do not cause bleeding by themselves, but they can amplify bleeding from a fissure or hemorrhoid that might otherwise be trivial. If you are on anticoagulation therapy and notice rectal bleeding, let your prescriber know even if you suspect the cause is benign, because the bleeding may be harder to control.
Oral iron supplements, as mentioned in the pregnancy section, are well documented to cause mucosal irritation in the upper gastrointestinal tract and frequently produce black stools and constipation.7PubMed. Iron-induced mucosal pathology of the upper gastrointestinal tract: a common finding in patients on oral iron therapy The constipation from iron then raises the risk of the same downstream problems: harder stool, more straining, more fissures. Switching to a different iron formulation or taking the supplement every other day instead of daily are strategies your doctor may suggest to reduce side effects while maintaining adequate iron absorption.
NSAIDs like ibuprofen and aspirin are another overlooked contributor. Regular NSAID use can erode the stomach and intestinal lining, and the resulting bleeding typically presents as dark or tarry stool rather than bright red blood. If you are taking NSAIDs daily and notice darkened stools, consider whether the medication rather than constipation is the source.