Anal fissures are small tears in the lining of the anal canal, and the most common cause is trauma from passing hard, dry stools. Straining during bowel movements stretches the tissue beyond its limit, creating a painful split. About 1 in 1,000 people develop a fissure each year, with the highest rates among adults aged 25 to 34.
Straining and Stool Consistency
The overwhelming majority of fissures start with a mechanical injury. Constipation is the leading trigger: large, hard stools force the anal canal to stretch more than it can handle, and the delicate lining tears. But diarrhea can cause fissures too. Frequent, loose stools irritate and weaken the tissue, making it vulnerable to tearing even without significant stretching.
Straining plays a central role regardless of stool type. When you bear down hard, you put intense pressure on the anal lining from both sides, the stool pushing outward and the surrounding muscles pulling inward. This is why anything that leads to prolonged time on the toilet, whether constipation, urgency, or simply habit, raises the risk.
The Muscle Spasm Cycle
Once a tear forms, the body’s response often makes things worse. Your anus is ringed by two circular muscles called sphincters that control bowel movements. When the lining tears, pain triggers these muscles to clench and spasm. That tightening does two harmful things: it pulls the edges of the tear apart, preventing them from knitting together, and it squeezes the tiny blood vessels that supply the area.
With less blood flow, the tissue gets less oxygen and fewer healing nutrients. This creates a self-reinforcing loop: the tear causes pain, the pain causes spasm, the spasm reduces blood flow, and poor blood flow keeps the tear from healing. This cycle is the main reason some fissures persist for weeks or months instead of resolving on their own. In many people with chronic fissures, the resting pressure of the internal sphincter muscle is measurably higher than normal, sustaining the ischemia (reduced blood supply) that blocks recovery.
Pregnancy and Childbirth
Anal fissures and hemorrhoids affect roughly 40% of pregnant women and new mothers. Several factors converge during pregnancy to make fissures more likely. As the uterus grows, it puts increasing mechanical pressure on the rectum and the veins that drain it, reducing blood circulation to the internal sphincter. Hormonal shifts, particularly rising progesterone, relax vein walls and alter tissue elasticity throughout the pelvic floor. Constipation, already common in pregnancy, compounds the problem.
Childbirth itself carries significant risk. Prolonged labor (over 12 hours), a long pushing stage, and delivering a baby weighing 4,000 grams (about 8.8 pounds) or more all increase the chance of perianal injury. Most pregnancy-related fissures appear either in the third trimester or within the first two days after delivery.
Where the Tear Forms Matters
Most fissures occur along the posterior midline, the back wall of the anal canal closest to the tailbone. A smaller number appear along the anterior midline, at the front. These locations are considered typical, and they reflect the mechanical stresses of normal bowel movements combined with the way blood supply is distributed in the anal canal. The posterior midline receives the least blood flow of any part of the anal lining, making it the most vulnerable spot.
Fissures that appear off to the side, in unusual positions, or in multiples are a different story. These atypical fissures often signal an underlying medical condition rather than simple mechanical trauma. Crohn’s disease is one of the most common culprits. Other possibilities include tuberculosis, HIV, sexually transmitted infections, anal cancer, radiation-related tissue damage, and leukemia. If you have a fissure that doesn’t heal with standard care or is located in an unusual position, that distinction matters because it changes what your doctor will look for next.
Underlying Conditions That Cause Fissures
Inflammatory bowel disease, particularly Crohn’s disease, deserves special attention. Crohn’s causes chronic inflammation that can affect any part of the digestive tract, including the anus. The fissures it produces tend to be deeper, wider, and more stubborn than ordinary ones. They also frequently appear in atypical locations. Unlike primary fissures where the sphincter muscle is abnormally tight, fissures caused by Crohn’s or other systemic diseases are often associated with normal or even reduced sphincter pressure, because the problem isn’t mechanical tension but rather tissue breakdown from inflammation or infection.
Prior anorectal surgery can also predispose someone to fissures. Scar tissue is less elastic than healthy tissue, and changes to the sphincter muscles from previous procedures can alter the mechanics of the anal canal in ways that make tearing more likely.
Fissures in Children
Children get anal fissures too, though less frequently than adults. The incidence in kids aged 6 to 17 is about half the rate seen in young adults. Constipation is the primary cause, just as in adults. Hard stools are painful to pass, and once a child experiences that pain, they often start holding in bowel movements to avoid it, which makes the stool even harder and larger, leading to more tearing. This withholding cycle can be especially difficult to break in toddlers and young children.
One common assumption is that formula-fed infants are more prone to fissures than breastfed babies, but research has found no difference in fissure rates between breastfed, formula-fed, and mixed-fed infants.
Age and Sex Patterns
Fissures can happen at any age, but the risk isn’t evenly distributed. In a large population study covering seven years of data, the peak incidence was among 25- to 34-year-olds at 1.76 cases per 1,000 people per year. Overall, women were slightly more affected than men (1.14 versus 1.04 per 1,000), but the gender gap shifts with age. Females aged 12 to 24 had significantly higher rates than males in the same age group, likely reflecting the added impact of hormonal changes and, in older teens and young adults, pregnancy. Among adults 55 to 64, the pattern reversed, with men showing higher rates than women.
The average age at diagnosis was about 41 for women and 47 for men. These are not conditions of old age or youth specifically; they cluster in the working-age years when the combination of diet, stress, sedentary habits, and (for women) reproductive factors converges.
How Fissures Are Identified
In most cases, a doctor can identify a fissure just by looking. A visual inspection of the anal area, sometimes with gentle separation of the buttocks, reveals the tear. A digital rectal exam is usually unnecessary and often too painful to perform when a fissure is present.
Further testing becomes important when the picture isn’t straightforward. If the fissure is in an atypical location, if there are multiple fissures, if there’s significant bleeding in someone at higher risk for colorectal cancer, or if the fissure refuses to heal with basic treatment, your doctor may recommend examination under anesthesia, imaging, endoscopy, or biopsy to look for underlying causes.