Causes of constipation | Cancer Research UK
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Why constipation happens

There are several possible causes of constipation. To understand them, it helps to know a bit about how your bowel (large intestine) works.

The large bowel draws water and nutrients into the body from the food you eat and drink. As the digested food passes through the bowel, the waste matter or stool gradually forms. The stool is stored in your rectum, or back passage, until it is ready to pass out of your body as a bowel motion.

The muscles in your bowel help to push the stools into your rectum. When it gets to your rectum the stools are solid. But if too much water is drawn back into the body from the bowel, or if the muscles in your bowel are weak and slow, your stools become hard and dry, causing constipation.


Cancer treatments

Treatments that can cause constipation include

Cancer drugs

Some chemotherapy drugs and biological therapy drugs cause constipation. This can be because the drug affects the nerve supply to the bowel for a while. Unfortunately, some anti sickness drugs and painkillers can make this worse.


Abdominal surgery

If you have surgery to your bowel, your bowel muscles may be weaker after the operation. Depending on the type of operation you have, this may only be temporary, but sometimes it can be permanent. This may make it more difficult to push stools out.

After any type of abdominal surgery your tummy may be too sore for you to want to push the stools out.

The drugs you have during a general anaesthetic and painkillers that you have after surgery can also cause constipation. If you have a very big operation you may not be able to eat or drink for a day or two after your operation, which makes constipation more likely.

Occasionally, some major pelvic operations can cause damage to the nerves that help the bowel to work properly.

If you have any problems opening your bowels after surgery your doctor or nurse will give you medicines to take called stool softeners.


The cancer itself

If a tumour is pressing on the nerves in your spinal cord it can slow down or stop the movement of your bowel and so cause constipation. Tumours in the abdomen can compress, squeeze, or narrow the bowel and back passage (rectum) making it difficult for you to have a bowel motion. Or a tumour in the lining of the bowel can affect the nerve supply to the muscles and cause constipation.


Side effects from other medicines

Constipation can be a side effect of many types of drugs. Doctors often use some of these drugs to help control cancer symptoms. They also use them to reduce the side effects of treatments such as chemotherapy, biological therapy and radiotherapy. If you need to take these drugs for a long period of time your doctor or nurse will also prescribe drugs to help prevent constipation.

The most common drugs to cause constipation in people with cancer are

Other drugs that can cause constipation are

  • Some blood pressure medicines
  • Anti depressants
  • Vitamin supplements, such as iron and calcium
  • Drugs to stop fits (anti convulsants or anti epileptics)
  • Drugs for Parkinson’s disease
  • Drugs to make you pass urine (diuretics)
  • Some medicines for indigestion (antacids)

Too little fibre in your diet

Fibre helps to keep our bowels working regularly and provides us with some nutrients too. People who eat plenty of foods high in fibre are less likely to become constipated. 

Fruit and vegetables

But if your cancer or cancer treatment is making you feel or be sick, you may find it hard to eat enough fibre.

There are tips on improving your diet to help with constipation in the managing constipation section.


Not drinking enough

Your body needs plenty of fluid to help stools stay soft and pass easily through your bowel. If you don’t drink enough, you will get dehydrated and begin to have problems with constipation. 


There are tips on how to increase your fluid intake in the managing constipation section.


Lack of exercise

If you find yourself unable to take much daily exercise, it can reduce muscle tone in your tummy (abdomen) and bowel (intestines). This slows down the movement of stools through your gut


If you can take regular gentle exercise, it will help to keep your bowels working properly.


Long term use of laxatives

Stool softeners and laxatives are medicines that help to treat constipation. Many of these can be bought over the counter without a prescription at your chemist or in health food shops. But it is very important to use them properly.

If you take laxatives regularly over a long period of time, they can make the bowel become lazy and not work properly. They can also damage the nerve cells in the bowel making it difficult for you to push stools out.

Always let your doctor know when you take any drugs to help your bowels. You may not actually need them and there are other ways of helping you to have regular bowel motions. You might just need to change your diet, drink more liquid or increase the amount of exercise you do.


Ignoring the urge to open your bowels

Sometimes people find it difficult to open their bowels away from the comfort and security of their own home. Others find that their busy lifestyle doesn’t allow them to go to the toilet to open their bowels at the right times. 

Research has shown that people who ignore the urge to open their bowels are more at risk of developing constipation. Water is drawn out of the stool as it sits in the rectum and so it gets harder and more difficult to pass. It is always best to listen to your body. If you feel the urge to have a bowel movement, then go as soon as you can.


Depression and anxiety

People with cancer sometimes suffer from depression and anxiety. These conditions can sometimes lead to constipation. Nerves link your brain and gut. When you are depressed, these nerves are not as active as usual. This can affect the way the muscles in your bowel work. There is information about depression and anxiety in the section about emotions and cancer.


Other medical conditions

Many medical conditions, other than cancer, can cause constipation. Some of these include

  • Bowel blockage (obstruction)
  • Parkinson’s disease
  • Spinal cord injuries
  • Diabetes
  • Having too much calcium in your blood (hypercalcaemia)
  • Multiple sclerosis
  • Thyroid problems
  • Having a stroke
  • Problems with the structure of the bowel and back passage (rectum)
  • Hirschsprung’s disease – a rare disease that is present from birth and affects the nerves in your bowel
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Updated: 21 May 2014