Possible side effects of radiotherapy for rectal cancer

The possible side effects of radiotherapy will depend on the type of cancer you have, the dose of radiotherapy and whether you’re having external or internal radiotherapy. Some possible side effects are:

  • tiredness
  • effects on the skin
  • effects on pubic hair
  • bowel changes
  • bladder changes.

Many side effects can be managed successfully and most will begin to get better a few weeks after treatment ends. Looking after yourself by not smoking, drinking plenty of water and doing some gentle exercise will help with your recovery.

Some people may have side effects that last for longer, or develop a while after they have finished their treatment. These are called long-term or late effects. Your cancer doctor, specialist nurse or radiographer will explain the most likely side effects in your situation. It’s important to tell them if you notice side effects aren’t getting better or new ones are developing. There are many things that can be done to help.

Side effects of treatment

Side effects depend on things such as the dose of radiotherapy, whether it’s external or internal, and whether you have chemotherapy as well (chemoradiation).

Side effects usually start a week or two after starting treatment. They may continue to get worse for a couple of weeks after treatment, before beginning to get better. Side effects usually improve gradually over the next few weeks or more.

Smoking can make side effects worse. If you smoke, try to give up smoking or cut down. Drink at least 2–3 litres of fluid a day. Water is best. Drinks containing caffeine and alcohol can make bowel and bladder symptoms worse.

It’s important to tell your radiographer, cancer specialist or specialist nurse if you have side effects. They can give you advice on how to manage them and prescribe treatments that can help.

It may take some time to recover, particularly after longer courses of radiotherapy or chemoradiation. Look after yourself by getting enough rest and gradually increasing your physical activity. This will help with your recovery.

Occasionally, some side effects don’t completely go away. Sometimes, side effects develop months or years later. These are called long-term or late effects. If side effects don’t get better or you notice new side effects developing, tell your cancer nurse or doctor. There are many things that can be done to help. We have more information about coping with late effects of treatment on your bowel.


Radiotherapy often makes people feel tired, especially towards the end of treatment. Tiredness may last for a few months. Your energy levels will then gradually improve.

We have more information about coping with fatigue.

Effects on the skin and pubic hair

Sometimes, radiotherapy can cause a skin reaction in the area being treated. The skin may redden or get darker, and become dry, flaky and itchy. Towards the end of treatment, the skin sometimes becomes moist and sore. There may be breaks in the skin, especially around the scrotum (men), vagina (women), back passage and groin.

Your radiographer or specialist nurse will tell you how to look after your skin. They will check your skin regularly. Tell them if it is sore or if you notice any other changes. They may prescribe a cream, dressings and painkillers to help.

Your pubic hair (hair around your genital area) may fall out. It should start to grow back again a few weeks after radiotherapy finishes, but it may be thinner. Occasionally, hair loss can be permanent.

Bowel side effects

You may have loose stools or constipation, or need to open your bowels urgently. Some people have cramping pains in their tummy or back passage, or have more wind than usual.

Diarrhoea usually starts during or after the second week of radiotherapy. If you have diarrhoea, your specialist will prescribe anti-diarrhoeal tablets, such as loperamide, to help.

If you have tummy cramps, let your cancer specialist or nurse know. They can prescribe anti-spasmodic or muscle relaxant drugs to help. If you’re constipated, your doctor will usually prescribe a laxative and you’ll be given advice on diet.

Let your nurse or radiographer know if you have any soiling or leakage. They will give you advice on coping with this and looking after the skin in the area.

You may be advised to make changes to your diet during radiotherapy. Bowel side effects usually start to improve about two weeks after radiotherapy has finished. Sometimes, it may take a few months.

Bladder irritation

Radiotherapy can cause irritation and inflammation of the bladder lining. You may need to pass urine more often (frequency) and have a burning sensation when you do. You may also feel that you can’t wait when you need to pass urine (urgency). There may be blood in your urine (haematuria).

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