What actually causes an anal fissure?
When patients ask "why did I get this?", the honest answer has two parts. Every fissure has a starting cause, the injury that tears the skin, and a maintaining cause, the reason the tear does not heal the way a paper cut on your finger would.
The starting cause is mechanical. The anoderm, the skin that lines the lower anal canal, is thin, tightly attached to the tissue below and has very little room to stretch. A stool that is too hard, too large or that passes with straining can exceed that limit and split it. So can repeated, irritating diarrhea, and so can the stretching of a vaginal delivery. Most fissures appear in the back midline, where the blood supply of the anal canal is weakest; a minority appear in the front midline, more often in women.
The maintaining cause is muscular. Underneath the tear lies the internal anal sphincter, a muscle you do not control. In most people with a chronic fissure it sits at a higher resting pressure than normal. That pressure squeezes the small vessels that feed the back of the canal, so the tear gets too little blood to close. Pain then tightens the muscle further. This is why you can fix the stool and still have a fissure that refuses to heal. The basics of the anatomy are in What is an anal fissure? A surgeon explains it with a drawing of the anal canal and on our causes page.
How does constipation cause a fissure?
Constipation is the leading cause, and it works in three ways at once:
- Hard, dry stools are less flexible and scrape the anoderm as they pass.
- Bulky stools stretch the anal opening beyond what the lining can tolerate.
- Straining raises pressure in the pelvis and forces the stool through a canal that is not fully relaxed.
The trap is what happens next. The first bowel movement after the tear hurts, so many people start postponing the toilet. Stool that stays longer in the colon loses more water, becomes harder, and the next movement tears the fissure again. Fear of the bathroom becomes a cause in itself; we explain how to break that loop in All guides.
Common contributors to constipation in our patients are a low-fiber diet, little water, long hours sitting, ignoring the urge because of work or travel, and medicines such as opioid painkillers, iron supplements and some antidepressants. Children get fissures for the same reason: hard stools after a change in diet, toilet training or holding in. In children, the treatment is mostly about the stool; a randomized trial in 48 children aged 14 or younger found that polyethylene glycol alone improved the fissure as much as polyethylene glycol combined with topical diltiazem at 12 weeks.
The flip side is good news: the cause most people have is the one they can control. In a classic trial, patients who kept taking unprocessed bran 5 g three times a day after an acute fissure had a 16 % recurrence at one year, compared with 60 % on a lower dose and 68 % on placebo (Jensen, 1987). For a practical plan, read All guides.
Can diarrhea cause an anal fissure?
Yes, and it surprises people who think fissures are only about hard stools. Diarrhea damages the anal canal differently:
- Frequency: six or eight bowel movements a day mean six or eight episodes of stretching and wiping.
- Chemistry: liquid stool carries bile acids and digestive enzymes that irritate the anoderm and make it fragile.
- Force: explosive bowel movements can tear the lining just as a hard stool does.
- Friction: repeated wiping with dry paper scrapes the already irritated skin.
Diarrhea that lasts a few days (an infection, a bad meal, a course of antibiotics) usually causes an acute fissure that heals once stools return to normal. Diarrhea that lasts weeks deserves investigation in its own right, because inflammatory bowel disease and other conditions can cause both the diarrhea and the fissure. If you have chronic diarrhea, abdominal pain or weight loss together with a fissure, see Anal fissure and inflammatory bowel disease: how is a Crohn’s fissure different?.
Why do some women get a fissure during pregnancy or after childbirth?
Pregnancy and the weeks after delivery combine several risk factors: hormones that slow the bowel, iron supplements, less physical activity, pressure from the uterus, and then the stretching of the perineum during a vaginal birth. After delivery, pain from stitches and fear of the first bowel movement add to the constipation.
The best prospective data come from a cohort of 280 pregnant women followed through pregnancy and after childbirth: 43.9 % developed some perianal disease, mostly in the third trimester, and constipation was by far the strongest risk factor (odds ratio 18.98). An isolated anal fissure, however, was uncommon in that cohort (0.71 %); most of the perianal disease was hemorrhoidal. In other words, pregnancy raises the risk, and constipation is the lever you can pull.
Fissures after childbirth are more often in the front midline, where the pressure of delivery concentrates. They also differ in one important way: some of these women have a sphincter that is not tight, or that was weakened by the birth, which changes which treatments are appropriate. We discuss this in Anterior anal fissure in women: why it happens and what changes in treatment and in All guides.
What is sphincter hypertonia, and why does it keep the fissure open?
Hypertonia means the internal sphincter rests at a higher pressure than it should. Studies using anal manometry, a test that measures pressure inside the canal, show that most people with a chronic fissure have this pattern. Some had a tight sphincter before the fissure; in others, the pain of the fissure itself drives the muscle into spasm. Either way the result is the same cycle:
Each step feeds the next. The tear hurts, the pain tightens the internal sphincter, the tight muscle chokes the small vessels at the back of the canal, and without enough blood the tear cannot heal, so it keeps hurting. A hard stool restarts the cycle every day.
This is the cause that medicines target. Topical nifedipine and diltiazem (calcium channel blockers) and nitroglycerin (a nitric oxide donor) lower the resting pressure of the sphincter for as long as they are used, which is why they need 6 to 8 weeks, two to three times a day. When they fail, botulinum toxin relaxes the muscle for a few months, and a lateral internal sphincterotomy lowers the pressure permanently. The treatment page compares each step.
The exception matters: not every fissure is a high-pressure fissure. Some fissures after childbirth, in older adults or after previous anal surgery occur with a normal or low sphincter pressure. Relaxing the muscle further is not the answer there, and a surgeon may suggest other approaches.
What other causes and risk factors should you know about?
Beyond the three big causes, these are the factors we ask about in clinic, from common to rare:
| Factor | How it contributes | Typical clue |
|---|---|---|
| Constipating medicines (opioids, iron, some antidepressants) | Harder stools and straining | Fissure that began after starting a new medicine |
| Anal intercourse or insertion of objects | Direct stretching of the anoderm | Pain starting after the event; often in the midline |
| Previous anal surgery (hemorrhoids, fistula) | Scarring and a less elastic canal | History of surgery; sometimes a narrow canal |
| Crohn’s disease or ulcerative colitis | Inflammation of the anal lining | Off-midline or multiple fissures, chronic diarrhea, weight loss |
| Infections (syphilis, herpes, HIV, tuberculosis) | Ulcers that mimic a fissure | Large, irregular or painless ulcers, other symptoms |
| Anal cancer and some blood diseases | Ulcer that looks like a fissure | Hard edges, a growing lump, no healing with treatment |
Crohn’s disease deserves attention because its fissures behave differently. In a series of 56 patients with Crohn’s disease, 26 % of fissures that did not heal progressed to an abscess or fistula (Fleshner, 1995). A fissure in an unusual position is the most important hint; we cover it in Lateral or multiple anal fissures: when should you think of Crohn’s, HIV, syphilis, tuberculosis or cancer?.
Stress does not tear the skin by itself, but it changes bowel habits toward constipation or diarrhea and increases muscle tension, so it can contribute.
When does the cause point to something more serious?
A typical fissure, with a clear trigger such as a hard stool and a tear in the back or front midline, rarely hides anything else. The cause needs a closer look when the story or the examination does not fit. See a doctor without delay if you notice:
- Bleeding after the age of 45, or blood mixed with the stool rather than on the paper, especially if you have never had a colonoscopy.
- A fissure off the midline, several fissures at once, or a large, irregular ulcer.
- Fever, pus or a hot, painful swelling next to the anus, which suggests an abscess.
- A lump that grows or feels hard.
- Unintended weight loss, chronic diarrhea or a change in bowel habit lasting weeks.
- No improvement after 8 weeks of correct treatment.
In these cases a colorectal surgeon examines the area, sometimes under sedation, and may take a biopsy or order a colonoscopy. Finding the real cause early changes the treatment completely.
Where does FISURIN fit?
FISURIN acts on the maintaining cause, not on the starting one. It combines nifedipine, which lowers the resting pressure of the internal sphincter, with lidocaine for pain, in a water-based hydrogel and a written 6-week protocol designed by our surgeons at Clínica Proctológica de Lima. It cannot fix constipation, diarrhea or an underlying disease: soft stools remain your job, and an atypical fissure still needs an examination. A randomized double-blind trial of nifedipine 0.3 % with lidocaine 1.5 % reported healing in 94.5 % of chronic fissures at 6 weeks, against 16.4 % with the control (Perrotti, 2002). Today FISURIN is a magistral formula prepared under prescription in Peru and is not shipped abroad; FISURIN Hydrogel and FISURIN Relief are in preparation, with a waiting list. See what is inside FISURIN or how to get it where you live.
Frequently asked questions
What is the most common cause of an anal fissure?
Constipation with hard or bulky stools, which tears the lining of the anal canal, usually in the back midline. Repeated diarrhea and childbirth cause most of the rest.
Why does my fissure not heal even though my stools are soft now?
Because the internal sphincter is often in spasm and reduces blood flow to the tear. Soft stools stop the re-tearing, but a chronic fissure usually also needs a sphincter-relaxing ointment for 6 to 8 weeks.
Can stress cause an anal fissure?
Not directly. Stress changes bowel habits toward constipation or diarrhea and increases muscle tension, and both favor a fissure.
Can spicy food cause an anal fissure?
Spicy food does not tear the skin, but it can make an existing fissure burn and, in some people, trigger loose stools. If it worsens your symptoms, reduce it while the fissure heals.
Are anal fissures a sign of cancer?
Almost never. A typical midline fissure is benign. A fissure off the midline, a hard lump, an irregular ulcer or one that does not heal after 8 weeks needs an examination to rule out other causes, including cancer.