Side effects of radiotherapy

Side effects tend to start a week 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 around 2 weeks or so.

Radiotherapy for advanced cancer aims to make you feel better. So your cancer specialist will try to choose treatments that have as few side effects as possible. 

The side effects vary from person to person. You might not have all of the effects mentioned.

Side effects can include:

You might feel tired during your treatment. It tends to get worse as the treatment goes on. You might also feel weak and lack energy. 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, such as exercise. Some research has shown that taking gentle exercise can give you more energy. It's 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 another type.   

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 passage (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. 

Tell your doctor or nurse about any side effects and they can find ways to help you.

Radiotherapy can cause different side effects when it treats different areas of the body.

Last reviewed: 
12 Nov 2018
  • Metastatic Colorectal Cancer: ESMO Clinical Practice Guidelines 
    E Cutsem and others
    Annals of Oncology, 2014. Volume 25, Pages ii1-iii9

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