Causes of constipation

To understand what causes constipation, it helps to know 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 colon), the waste matter or stool (poo) gradually forms.

The poo is stored in your rectum, or back passage, until it is ready to pass out of your body as a bowel motion.

Diagram of the bowel

The muscles in your bowel help to push the poo into your rectum. When it gets to your rectum the poo is solid. But you can become constipated if:

  • too much water is drawn back into the body from the bowel
  • the muscles in your bowel are weak and slow

Cancer treatments

Some cancer treatments can cause constipation, including some cancer drugs, and having surgery to your tummy (abdomen). 

Cancer drugs

Some chemotherapy drugs and targeted cancer 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.

Surgery to your tummy (abdomen)

After surgery to your bowel, your bowel muscles might be weaker. This may be temporary, but sometimes it can be permanent – this depends on the type of operation you have.

After this type of surgery, there are several factors that can cause constipation:

  • weak muscles may make it more difficult to push the poo out
  • your tummy might be too sore for you to push out the poo
  • the drugs you have as an anaesthetic or for pain can also cause constipation
  • 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 damage the nerves that help the bowel to work properly

The cancer itself

A tumour that presses on the nerves in your spinal cord can slow down or stop the movement of your bowel. This causes constipation.

Tumours in the tummy (abdomen) can squash, 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. Some of these drugs help control cancer symptoms or reduce side effects from treatments.

Your doctor or nurse will also prescribe medicine to help prevent constipation.

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

  • painkillers, especially morphine based drugs (opioids)
  • anti sickness medicines

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

Dietary fibre is a word used to describe plant based carbohydrates. They are different to other carbohydrates (such as starch or sugar) as they are not digested in the small intestine and reach the large intestine (colon). 

Fibre helps to keep our bowels working regularly. If you eat plenty of foods high in fibre you're less likely to become constipated. 

But if you feel sick, you might find it hard to eat enough fibre.

Not drinking enough

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

Lack of exercise

Not getting much daily exercise can reduce muscle tone in your tummy (abdomen) and bowel (intestines). This slows down the movement of poo through your gut. 

Taking regular gentle exercise will help keep your bowels working properly. 

Long term use of laxatives

Laxatives and stool softeners are medicines for constipation. You can often buy these over the counter without a prescription. But it is very important to use them properly.

Taking laxatives regularly over a long period of time 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 drugs to help your bowels. You might not actually need them and there are other ways of helping you to have regular bowel motions. For example, changing your diet, drinking more liquid or increasing the amount of exercise you do.

Ignoring the urge to open your bowels

Some people find it difficult to open their bowels away from the comfort of their own home. Others find that a busy lifestyle stops them opening their bowels. 

Research has shown people who ignore the urge to open their bowels are more at risk of constipation. Water is drawn out of the poo 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. Go to the toilet when you feel the urge to have a bowel movement.

Depression and anxiety

People with cancer may 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.

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
  • high calcium levels in the blood (hypercalcaemia)
  • low levels of potassium in the blood (hypokalaemia)
Last reviewed: 
08 Aug 2019
  • Constipation - Clinical Knowledge Summary 
    National Institute for Health and Care Excellence (NICE), 2016

  • Diagnosis, assessment and management of constipation in advanced cancer: ESMO Clinical Practice Guidelines

    P.J Larkin and others

    Annals of Oncology, 2018. Vol 29, Supplement 4.

Related links