Bowel changes after bowel cancer treatment

Late effects are side effects that do not go away, or that start months or years after treatment. If you have bowel late effects after treatment for colon, rectal or anal cancer, there are things that can help.

About bowel late effects

Late effects are side effects that do not go away, or that start months or years after treatment. The most common late effect of bowel cancer treatment is a change in how the bowel works.

Most people have bowel changes after surgery or radiotherapy for colon, rectal or anal cancer (bowel cancer).

Your bowel habits may not go back to the way they were before treatment. But over time, bowel symptoms usually become less troublesome. Most people find they get into a new routine that becomes normal for them. This can take a year or more.

If you have ongoing bowel problems, or any of the symptoms listed below, there are many things that can help. Your doctor or specialist nurse can give you advice.

Symptoms of bowel changes

Possible symptoms of late effects to the bowel include:

  • bleeding from the back passage (after radiotherapy only)
  • passing mucus (a clear, sticky substance)
  • cramps or spasms in the bowel, which may be painful
  • difficulty emptying the bowel completely and needing to return to the toilet quickly
  • feeling that you need to pass stools (poo) even when your bowel is empty – this is called tenesmus
  • diarrhoea or constipation
  • needing to rush to empty your bowels (urgency)
  • problems controlling your bowel, causing leakage or soiling (bowel incontinence)
  • passing a lot of wind or losing control of passing wind.


Many people find changing their diet and taking anti-diarrhoea drugs stops the diarrhoea. But if this does not help, ask your doctor to refer you to a gastroenterologist. Lots of different things can cause diarrhoea, and a gastroenterologist will be able to do a full assessment.

After bowel cancer treatment, particularly pelvic radiotherapy, some people have diarrhoea caused by changes to the small bowel, such as:

  • bile acid malabsorption (also called bile acid diarrhoea)
  • small bowel bacterial overgrowth.

Bile acid malabsorption

Bile acids are made in the liver and go to the small bowel when you eat to help digest fats. When the bile acids reach the far end of the small bowel, they are absorbed back into the body.

Sometimes, radiotherapy for bowel cancer damages the small bowel. Or, part of the small bowel may be removed during surgery. If the small bowel cannot re-absorb the bile acids, this can cause watery diarrhoea, painful cramping and bloating. This is called bile acid malabsorption or bile acid diarrhoea.

Your doctor may advise you to start the following treatments, to see if symptoms improve:

  • Eating a low-fat diet – a dietitian will help you do this in a balanced way.
  • Taking anti-diarrhoea drugs.
  • Taking drugs that reduce the effect of bile acids on the bowel – the drug most commonly used is colestyramine (Questran®, Questran Light®). It is a powder that you mix with water or fruit juice. If colestyramine does not work, your specialist may prescribe a tablet called colesevelam (Cholestagel®).

A scan called a SeHCAT scan can help diagnose bile acid malabsorption. Your doctor can explain more about this test. SeHCAT scans are not widely available and may not be needed.

Most people with bile acid malabsorption do not absorb enough vitamins and need to take vitamin supplements. Your doctor can talk to your more about this.

Small bowel bacterial overgrowth

The large bowel contains lots of healthy or ‘good’ bacteria, which help digest food. But a healthy small bowel contains almost no bacteria. After pelvic radiotherapy, sometimes this changes and bacteria are found in the small bowel. This is called small bowel bacterial overgrowth, or SIBO. It can cause symptoms, including:

  • diarrhoea
  • wind
  • bloating
  • constipation
  • feeling sick
  • omiting
  • bad breath.

A breath test can help to find out whether you have small bowel bacterial overgrowth. You have the test as an outpatient. Samples of your breath are tested for signs of bacteria in your small bowel.

Small bowel bacterial overgrowth is usually treated with antibiotics.


If you have problems with constipation after treatment, the following tips may help:

  • include more fibre in your diet
  • drink at least 1 to 2 litres (2 to 3½ pints) of fluid a day
  • do daily exercise, such as walking
  • get into a toilet routine
  • use the correct toilet posture (sit on the toilet in the right position)
  • check with your doctor if you are taking medicines that can cause constipation
  • take medicines to treat constipation.

If the constipation gets worse or you have severe tummy (abdominal) pain, get advice from your doctor or nurse.

We have more about how to manage bowel problems, such as constipation. This includes information about diet, toilet routine, toilet posture and medicines and constipation.


Tenesmus is the feeling that you need to go to the toilet but your bowel is empty. It can involve straining, pain and cramping.

Tell your doctor or nurse if you have these symptoms.

We have more information about tenesmus.

Difficulty emptying the bowel

After rectal surgery, some people have problems emptying their bowel completely. Signs of this can include:

  • feeling there is still something in your bowel after you have passed a stool (poo)
  • having smaller, pellet-like stools (fragmented)
  • leakage of stool after you go to the toilet
  • needing to go back to the toilet several times after a bowel movement.

What can help will depend on the exact cause of your symptoms. Your doctor or continence specialist may suggest:

  • a toilet routine
  • using the correct toilet posture (sitting on the toilet in the right position)
  • pelvic floor exercises
  • changes to your diet
  • medicines to treat constipation
  • anti-diarrhoea medicines if your stools are too soft
  • bowel or colostomy irrigation.

We have more information about how to manage bowel problems. This includes information about diet, toilet routine, toilet posture and medicines that may help.

Bowel or colostomy irrigation

This is a way of emptying the bowel by introducing warm water into it. You should only use bowel or colostomy irrigation if it is recommended by a health professional.

Bowel or colostomy irrigation means you can empty your bowel completely at a time that suits you. You will be less likely to have leakage or incontinence at other times. If you have a colostomy, it can mean less worry that your stoma will be active at inconvenient times.

You place a narrow, flexible tube (catheter) into your back passage or stoma. You then put water into the bowel through the tube. You usually do this every day or every other day. It takes about half an hour.

Your continence specialist or stoma nurse can tell you more about bowel or colostomy irrigation. If it is suitable for you, they can teach you how to do it. They will also arrange for you to get the equipment you need on prescription.

Some people find bowel or colostomy irrigation too time-consuming. But others feel more in control of their bowel and more confident as a result.


If you have problems with wind after bowel cancer treatment, the following tips may help:

  • Cut down on foods and drinks that are causing wind.
  • Eat your meals at the same times each day.
  • Do not eat and drink at the same time.
  • Use pelvic floor exercises to strengthen the muscles used for bowel control.
  • Ask your doctor for advice if you take medicines that cause wind, such as Lactulose® or Fybogel®.
  • Try taking peppermint oil or eating live yoghurts. Or you could try taking charcoal tablets, which you can buy from the chemist. However, charcoal can affect other medicines you may be taking so always ask your doctor or pharmacist first.

Tell your doctor if this symptom is a problem. Sometimes other things may be making wind worse. For example, constipation or bowel conditions, such as diverticular disease can make wind worse. Wind can also be a symptom of a food intolerance, or a condition called small bowel bacterial overgrowth after radiotherapy.

Bleeding from the rectum

Bleeding from the back passage (rectum) is common after pelvic radiotherapy. Most people who have this side effect only notice bleeding occasionally. For a few people, bleeding can be heavy and needs treatment.

We have more information on bleeding from the rectum after radiotherapy.

Sore or itchy skin

Ongoing diarrhoea or leaking (incontinence) from the bowel can make the skin around the anus sore. Sometimes radiotherapy for rectal or anal cancer can also make this area of skin sore, red or broken.

If your skin is sore or passing a bowel movement is painful, speak to your doctor or nurse. They can give you advice about looking after your skin and may give you creams or ointments to use. They can also check your skin for signs of other problems such as piles (haemorrhoids) or fissures.

Anal fissure

After radiotherapy, the skin of the anus may become narrower and less stretchy. Sometimes a split develops in the skin of the anus. This is called an anal fissure. It can cause a sharp, intense pain when you pass a stool (poo).

We have more information about anal fissures after pelvic radiotherapy.

Rarer bowel problems


Normally, tissues and organs inside the tummy (abdomen) are slippery and move easily as the body moves. After surgery in the abdomen, bands of scar tissue (called adhesions) may form between abdominal tissues and organs, sticking them together.

Adhesions often do not cause any problems. But sometimes they can cause pain.

Rarely, adhesions can make part of the bowel twist or kink, pulling it out of place so that it becomes blocked.

We have more information about adhesions.


A fistula is an opening that forms between areas of the body that are not usually connected. Rarely after pelvic radiotherapy or surgery for rectal or anal cancer, an opening can develop between:

  • the rectum and vagina
  • the rectum and bladder or urethra.

Sometimes a fistula closes on its own. It can then be managed with treatment to control symptoms. If this does not happen, it may be possible to have an operation to close it.

We have more information about fistulas.

About our information

  • References

    Below is a sample of the sources used in our late effects of bowel cancer treatment information. If you would like more information about the sources we use, please contact us at

    Andreyev HJN, Muls AC, Norton C, et al. Guidance: The practical management of the gastrointestinal symptoms of pelvic radiation disease. Frontline Gastroenterology, 2015; 6, 53-72. 

    NICE. Faecal incontinence in adults: management (CG49). (Internet), 2007. Available from (accessed January 2021).

    NICE. Lower urinary tract symptoms in men: management (CG97). (Internet), 2015. Available from (accessed January 2021).

    NICE. Urinary incontinence and pelvic organ prolapse in women: management (NG123). (Internet), 2019. Available from (accessed January 2021).

  • Reviewers

    This information has been written, revised and edited by Macmillan Cancer Support’s Cancer Information Development team. It has been reviewed by expert medical and health professionals and people living with cancer. It has been approved by Chief Medical Editor, Professor Tim Iveson, Consultant Medical Oncologist.

    Our cancer information has been awarded the PIF TICK. Created by the Patient Information Forum, this quality mark shows we meet PIF’s 10 criteria for trustworthy health information.

Date reviewed

Reviewed: 01 July 2021
Next review: 01 July 2024
Trusted Information Creator - Patient Information Forum
Trusted Information Creator - Patient Information Forum

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