Anal fissures happen when the thin, delicate lining of the anal canal tears, usually from passing a hard or large stool. They’re one of the most common causes of anal pain and bleeding, and while most heal on their own within a few weeks, some become chronic. Understanding what causes them helps you avoid the triggers that lead to recurrence.
The Most Common Cause: Hard Stools and Straining
The inside of your anal canal is lined with a soft mucous membrane, similar to the lining of the rest of your large intestine. This tissue is thinner and more delicate than the skin on the outside of your body, which makes it vulnerable to tearing when it’s stretched too far. Passing a large, hard stool forces the anal canal open beyond its comfortable range, and the resulting stretch rips the lining. Straining during a bowel movement adds even more mechanical force.
Constipation is the single biggest driver. When stool sits in the colon too long, the body absorbs more water from it, leaving it dry, bulky, and difficult to pass. A low-fiber diet, not drinking enough water, and sedentary habits all contribute to this cycle. On the other end of the spectrum, chronic diarrhea can also cause fissures. Frequent loose stools irritate the anal lining, and the repeated wiping and inflammation weaken the tissue over time.
Why Some Fissures Won’t Heal
A small tear in such a well-supplied area should, in theory, heal quickly. But anal fissures have a unique problem: the internal anal sphincter, the ring of muscle that keeps the anal canal closed at rest, often goes into spasm after the initial injury. This spasm does two things. It causes sharp pain with every bowel movement, which makes people clench and avoid going to the bathroom, worsening constipation. And it compresses the tiny blood vessels that supply the injured tissue, cutting off the blood flow needed for healing.
Most 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. In people whose internal sphincter muscle is unusually tight or thickened, that blood supply is even more compromised, creating a zone that’s essentially starved of oxygen and nutrients. This is the main reason fissures can persist for months, even after the original cause is gone. The tear reopens with each bowel movement, the sphincter spasms again, blood flow drops further, and the cycle repeats.
Childbirth and Postpartum Fissures
Vaginal delivery is a significant cause of anal fissures and sphincter injuries, particularly for first-time mothers. The pressure of the baby’s head passing through the birth canal can stretch or tear the anal sphincter and surrounding tissue. About 5.7% of women delivering their first baby vaginally experience obstetric anal sphincter injuries, and this rate jumps to over 10% in women attempting vaginal birth after a previous cesarean. Women who have delivered before have a much lower rate, around 1.3%.
Several factors increase the risk during delivery: a baby weighing more than about 8.8 pounds, a head circumference of 35 centimeters or more, a prolonged pushing stage, the use of forceps, and the lithotomy position (lying on your back with legs elevated), which is the most common delivery position in North American hospitals. Fissures that develop during childbirth can overlap with other perineal tears, making them harder to identify and treat in the immediate postpartum period.
Inflammatory Bowel Disease
Crohn’s disease is a well-known cause of anal fissures. The chronic inflammation that defines Crohn’s can directly damage the anal lining, and the frequent diarrhea associated with flares adds ongoing irritation. Fissures from Crohn’s tend to behave differently from typical fissures. They may appear in unusual locations (off the midline or in multiple spots), be deeper, and resist standard treatments. Ulcerative colitis rarely causes anal fissures, since the inflammation in that condition typically doesn’t extend to the anal canal.
Location matters diagnostically. About 90% of fissures in men and women occur along the posterior midline, with about 10% of women and 1% of men having anterior midline fissures. When a fissure shows up on the side of the anal canal, or when there are multiple fissures at once, it raises suspicion for an underlying condition like inflammatory bowel disease, an infection, or immune suppression.
Sexually Transmitted Infections
Some infections can cause ulcers or tears in the anal area that look and feel like fissures. Herpes (HSV) and syphilis are the most common infectious causes of anal ulcers in sexually active people. Less frequently, conditions like chancroid and lymphogranuloma venereum can present in a similar way. These infection-related fissures typically don’t respond to the usual treatments for a standard fissure, which is often what prompts further testing. If a fissure appears in an atypical location, doesn’t heal as expected, or is accompanied by other symptoms like fever or discharge, testing for STIs and HIV is an important step.
Fissures in Babies and Young Children
Anal fissures are surprisingly common in infants and toddlers, and constipation is almost always the cause. Babies transitioning from breast milk to formula, or toddlers starting solid foods, often experience harder stools as their digestive systems adjust. The anal lining in young children is just as delicate as in adults, and a single hard stool can create a painful tear. The child then associates bowel movements with pain, starts holding stool in, and the resulting constipation makes the next movement even harder and more likely to reopen the tear.
Bright red blood on the stool or on a diaper is usually the first sign parents notice. In most cases, increasing fiber through fruits, vegetables, and adequate fluids breaks the cycle. The vast majority of pediatric fissures heal without any intervention beyond softening the stool.
Other Risk Factors
Anal intercourse can cause fissures through direct mechanical trauma to the anal lining, particularly without adequate lubrication. Aging also plays a role. As people get older, blood flow to the anal canal decreases and the tissue loses some of its elasticity, making tears more likely and slower to heal. Obesity increases the likelihood of constipation and straining. Prior anal surgery can leave scar tissue that’s less flexible than the original lining.
Even after a fissure heals completely, the area remains vulnerable. A single hard bowel movement can reopen the same spot. This is why long-term prevention focuses on keeping stools soft and easy to pass through a high-fiber diet (or fiber supplements like psyllium husk), staying well hydrated, and not delaying bowel movements when the urge arises. The goal is to remove the mechanical stress that caused the tear in the first place, so the lining can stay intact.