Possible side effects of radiotherapy for rectal cancer

The possible side effects of radiotherapy will depend on the dose of radiotherapy and whether you are 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 is important to tell them if you notice side effects are not getting better or new ones are developing. There are many things that can be done to help.

Side effects of radiotherapy

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

Side effects usually begin a week or two after starting treatment. They may continue to get worse for a few 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 or cut down. Drink at least two to three litres of fluid a day. Water is best. Drinks containing caffeine and alcohol can make bowel and bladder symptoms worse.

It is 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 do not completely go away. Sometimes side effects develop months or years later. These are called long-term or late effects. If side effects do not 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.


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.

Effects on the skin and pubic hair

Radiotherapy can sometimes 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 back passage, the groin, the scrotum in men and the vagina in women.

Your radiographer or specialist nurse will tell you how to look after your skin. They will also check your skin regularly. Tell them if it is sore or if you notice any other changes. They may prescribe 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, tell your cancer specialist or nurse. They can prescribe anti-spasmodic or muscle relaxant drugs to help. If you are constipated, your doctor will usually prescribe a laxative and you will be given advice on diet.

Tell your nurse or radiographer if you have any soiling or leakage. They will give you advice on coping with this and on looking after the skin in that 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 and have a burning sensation when you do. You may also feel that you cannot wait when you need to pass urine. There may also be blood in your urine. This is called haematuria.

Additional side effects in women

Changes to the vagina

Radiotherapy can make the lining of the vagina sore and inflamed. You may be advised not to have sex during treatment and for a few weeks after. This is to allow any inflammation or side effects to settle. Ask your doctor or nurse for advice.

If you do have sex during treatment, it is very important to use effective contraception to prevent a pregnancy. Radiation may cause damage to a baby conceived during or shortly after radiotherapy.

After radiotherapy, the vagina may be narrower, less stretchy and drier than before. This may make sex uncomfortable. Your specialist nurse may recommend you use vaginal dilators to try to prevent the vagina from narrowing. Dilators are tampon-shaped, plastic tubes of different sizes that you use with a lubricant.

Vaginal dryness can be relieved with vaginal lubricants or creams. Hormone creams can also help with dryness and vaginal narrowing. These are available on prescription from your doctor.

Early menopause and infertility

If you are still having menstrual periods, radiotherapy to the pelvic area will cause the menopause. The menopause means your ovaries are no longer producing eggs, so you will not be able to get pregnant.

Hormone replacement therapy (HRT) replaces the hormones your ovaries can no longer produce. This can improve menopausal symptoms, but it cannot prevent infertility.

Additional side effects in men


You can have sex during radiotherapy if you want to. Sperm produced during treatment and for some time after may be damaged but still fertile. This could cause abnormalities in a child conceived soon after radiotherapy. To prevent this, your doctors may recommend that you use contraception during treatment and for six months or more after it.

Radiotherapy can damage nerves in the pelvic area and blood vessels that supply blood to the penis. This can cause problems getting or keeping an erection (erectile dysfunction). Your cancer specialist will discuss this with you.

Some men may have a sharp pain when they ejaculate. This is because radiotherapy can irritate the tube that runs through the penis (the urethra). The pain should get better a few weeks after treatment finishes.


Radiotherapy may make you unable to father children (infertile). Your doctor or specialist nurse can talk to you about this.

For some men, it may be possible to have sperm stored before the treatment starts. This is called sperm banking. The sperm can then be used in the future. It is important to talk to your doctor or nurse before your treatment starts. They can advise you about sperm storage.

Back to Radiotherapy explained

Who might I meet?

You will meet many different specialists before, during and after radiotherapy treatment.