Anal fissure - Causes 

Causes of anal fissure 

Anal fissures are most commonly caused by damage to the anus or the anal canal. Most cases first develop when a particularly hard or large stool damages the anal canal.

The main causes of anal fissures include:

  • constipation – which can cause people to strain when passing a stool
  • diarrhoea – particularly reoccurring (chronic) diarrhoea
  • inflammatory bowel disease (IBD) – such as Crohn’s disease and ulcerative colitis
  • pregnancy – your risk of developing an anal fissure is increased during the third trimester of pregnancy (from week 29 to birth)
  • childbirth – the continual straining of your muscles during childbirth can cause the lining of your anus to tear 
  • sexually transmitted infection (STI) –  if the lining of the anus becomes infected it can cause the tissue to break down, leading to an anal fissure 

STIs known to affect the anus include:

  • syphilis – a bacterial infection that causes a wide range of symptoms
  • human papilloma virus (HPV) – the virus that causes genital warts
  • herpes – a viral infection that can cause blistering around the genitals
  • chlamydia – a bacterial infection that can cause pain when passing urine but often shows no symptoms

Cancer of the anus

In very rare cases, an anal fissure can be a sign of cancer. Your GP will be able to tell if your fissure is a sign of a more serious illness from your symptoms and the appearance of your fissure. They will refer you for further testing if they think it is necessary. It is important to remember that in the majority of cases, anal fissures are not caused by cancer.

Macmillan Cancer Support’s website has more information about anal cancer.

Chronic anal fissures

Some people may have underlying problems with the ring of muscle that surrounds their anal canal, known as the internal sphincter. These problems can prevent the anal canal from healing in the normal way, leading to the development of chronic anal fissures.

It is thought that some people have a sphincter muscle that is larger than normal, which produces muscle tension stronger than normal. The increased muscle tension causes increased pressure in the anal canal, making it more susceptible to tearing.

The increased pressure in the anal canal also restricts the blood flow to the anus. Reduced blood flow prevents your body from being able to heal itself as well as it usually can, making it difficult for your fissure to heal naturally. Furthermore, each time that you pass a stool, the injured tissue inside the anal canal is further damaged. This may be why some people develop chronic anal fissures.

Another common problem is that some people try to avoid passing stools due to the pain of an anal fissure. This can trigger the symptoms of constipation, which means that the fissure will not have a chance to heal.


Page last reviewed: 23/05/2012

Next review due: 23/05/2014

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Sexually transmitted infections (STIs)

Symptoms to look out for, where to go for help and how to protect yourself. Plus real-life stories from people with infections