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Side effects of radiotherapy

Find out about the side effects of external radiotherapy for cancer of the back passage (rectal cancer) and how to cope with them.

Side effects tend to start a few days after the radiotherapy begins. They gradually get worse during the treatment and for a couple of weeks after the treatment ends. But they usually begin to improve after 2 weeks or so.

Everyone is different and the side effects vary from person to person. You may not have all of the effects mentioned.

Side effects can include:

You are likely to feel very tired during your treatment. It tends to get worse as the treatment goes on. You might also feel weak and lack energy.

After a while you might need to sleep after each radiotherapy session. Rest when you need to.

Tiredness can carry on for some weeks after the treatment has ended. But it usually improves gradually.

Various things can help you to reduce tiredness and cope with it: for example, exercise. Some research has shown that taking gentle exercise can give you more energy. It is important to balance exercise with resting.

You might feel sick at times. You can have anti sickness medicines. Let your treatment team know if you still feel sick, as they can give you other medicines.   

Tell your doctor or nurse if you have diarrhoea. They can prescribe medicine to help you. 

Drink at least 2.5 litres of fluid a day. This helps to keep you hydrated.

Ask your nurse about soothing creams to apply around your back passage (rectum). The skin in that area can get very sore and even break if you have severe diarrhoea.

Contact your doctor or nurse immediately if you have diarrhoea 4 or more times a day, or any diarrhoea at night.

You may feel:  

  • as if you want to pass urine all the time (but when you go, there isn't much there)
  • as if you have a bladder infection (cystitis)
  • a burning pain when you do pass urine

Try to drink plenty of water. Many people think that drinking cranberry juice can be helpful with bladder problems. But cranberry juice can increase the effects of warfarin (a blood thinner or anticoagulant). You should not drink cranberry juice if you are taking warfarin. 

Your bladder inflammation should settle down after the treatment is over.  

Tell your doctor if you have any pain when passing urine. It could be a sign that you have an infection. You may need antibiotic treatment.

The skin around your anus and back pasage (rectum) is very sensitive. Radiotherapy can make it red and sore. 


  • Wash your skin with tepid water and simple soaps.
  • Pat your skin dry with a soft towel.
  • Ask your doctor, nurse or radiographer for creams to protect your skin and help it heal quickly.
  • Don't use perfumed or medicated soaps and lotions.
  • Try using a soft cushion if you have difficulty sitting comfortably for a while.

After your treatment is over, the soreness should gradually get better over a few weeks. 

Long term side effects

Most side effects gradually go away in the weeks or months after treatment. But some side effects can continue or might start some months or years later.  

Side effects if you have chemotherapy with radiotherapy

Chemotherapy combined with radiotherapy can make some side effects worse. Combining these treatments is called chemoradiotherapy.

Last reviewed: 
17 Sep 2015
  • Metastatic Colorectal Cancer: ESMO Clinical Practice Guidelines

    Van Cutsem (and others)
    Annals of Oncology, 2014. Vol 25, Issue 3

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