Anal fissures are small tears in the lining of the anal canal, and the most common cause is passing large or hard stools. Constipation and straining during bowel movements are the primary triggers, but chronic diarrhea, childbirth, and anal intercourse can also tear this delicate tissue. In less common cases, fissures signal an underlying condition like Crohn’s disease or an infection.
Constipation, Straining, and Hard Stools
The anal canal is lined with thin, sensitive tissue that tears relatively easily under mechanical stress. When you’re constipated and push hard to pass a bowel movement, or when the stool itself is unusually large or firm, that force can split the lining. This is by far the most frequent cause of anal fissures.
What surprises many people is that chronic diarrhea causes fissures too. Frequent, loose bowel movements irritate and weaken the anal lining over time, making it vulnerable to tearing. So the problem isn’t limited to one end of the spectrum. Anything that repeatedly stresses the anal canal, whether from straining or from sheer frequency, can lead to a fissure.
Why Fissures Get Stuck in a Pain Cycle
A fissure that doesn’t heal within a few weeks often gets trapped in a self-reinforcing loop. When the tear forms, the internal anal sphincter (the ring of muscle that keeps the anal canal closed) tends to tighten and spasm. That spasm does two things: it causes sharp pain on its own, and it squeezes the blood vessels that supply the injured area.
About 85 to 90 percent of fissures occur along the posterior midline, the back wall of the anal canal. This spot already has the poorest blood supply of any part of the canal. When the sphincter tightens, blood flow drops even further, starving the tissue of what it needs to repair itself. This is why so many fissures become chronic, lasting months instead of healing on their own within a couple of weeks. The pain of each bowel movement triggers another round of spasm, reduced blood flow, and delayed healing.
Childbirth and Physical Trauma
Vaginal delivery is a well-known trigger for anal fissures. During labor, the intense pressure and sudden contraction of the anal area can tear the lining directly. Many new mothers also develop constipation in the postpartum period due to hormonal changes, pain medications, or simply the fear of straining after delivery, which compounds the problem. The combination of birth trauma and subsequent hard stools makes the weeks after delivery a particularly high-risk window.
Anal intercourse is another direct cause. The mechanical stretching of the anal canal beyond its comfortable range can tear the lining in the same way a hard stool would.
Inflammatory and Infectious Causes
Most fissures are straightforward mechanical injuries. But when a fissure appears in an unusual location (off to the side rather than the front or back midline), doesn’t heal with standard treatment, or occurs alongside other symptoms, it may point to something else entirely.
Crohn’s disease is one of the more common underlying conditions linked to atypical fissures. The chronic inflammation it causes throughout the digestive tract can weaken the anal lining and impair healing. Ulcerative colitis can do the same. Certain infections, including syphilis, tuberculosis, and HIV, are also associated with fissures that look or behave differently from typical ones. These atypical fissures may be multiple, painless, or positioned off the midline. In rare cases, anal cancer or leukemia can present this way.
A fissure that won’t heal after proper treatment, or one that sits in an unusual spot, is worth a closer look from a specialist to rule out these less common causes.
How Fissures Differ From Hemorrhoids
People often confuse anal fissures with hemorrhoids because both involve pain and bleeding around the anus, but they’re distinct problems. Hemorrhoids are swollen veins. They typically show up as lumps or swelling and often cause itching, but most hemorrhoids don’t cause significant pain. Fissures are actual tears in the skin. They tend to produce sharp, cutting pain during bowel movements and sometimes a burning sensation that lingers afterward. Bright red blood on the toilet paper is common with both conditions, but if the dominant symptom is pain rather than a visible lump, a fissure is the more likely culprit.
Risk Factors That Make Fissures More Likely
Several factors raise your odds beyond the obvious triggers:
- Low fiber intake. Most Americans eat only 10 to 15 grams of fiber per day, roughly half the recommended 25 to 35 grams. Insufficient fiber produces hard, compact stools that are difficult to pass.
- Not drinking enough water. Fiber works by absorbing water to bulk and soften stool. Without adequate fluid (aim for 8 to 10 glasses per day), adding fiber can actually make stools harder. Caffeinated drinks don’t count toward this total because caffeine acts as a mild diuretic.
- Previous fissures. Once the tissue has torn, scar tissue forms during healing. Scar tissue is less flexible than the original lining, which makes the area more prone to tearing again.
- Age extremes. Infants and older adults develop fissures more frequently, infants because of immature digestive patterns and older adults because of reduced blood flow to the area.
Preventing Recurrence
Because most fissures stem from hard stools and straining, prevention centers on keeping bowel movements soft and easy to pass. The target is 25 to 35 grams of fiber daily from fruits, vegetables, whole grains, and legumes, which typically produces one or two soft, formed stools per day. Pair that with enough water to keep the fiber hydrated and effective.
Avoiding prolonged sitting on the toilet helps too. The longer you sit and strain, the more pressure builds in the anal canal. If a bowel movement isn’t happening within a few minutes, it’s better to get up and try again later than to push through it. Staying physically active also supports regular digestion, reducing the constipation episodes that start the whole cycle.