Causes of cancer fatigue

Many things can cause fatigue in people with cancer. This includes the cancer itself and the type of treatment you might have.

Doctors don't fully understand what causes cancer related fatigue. But we know that a number of things can make it worse. Fatigue is often worse in people who:

  • are having a combination of treatments
  • have advanced cancer
  • are elderly

It's important that you know you may be more likely to suffer from fatigue if you fall into one of these groups. 

Tell your doctor or nurse if you have any signs of fatigue so that they can help you manage it.

What could cause fatigue?

Cancer can cause you to feel tired due to a number of different things. This could be due to the cancer itself, low levels of red blood cells (anaemia), pain and other health problems such as depression.

The cancer itself

A cancer that affects your hormone levels could cause fatigue. Certain cancers such as breast and prostate cancer can change the levels of hormones in your body. This can cause a number of side effects including fatigue.

People with advanced cancer are more likely to have fatigue than those with earlier staged cancer. This could be because there are more cancer cells in the body, so the cancer itself is causing you to feel tired. You might also:

  • eat less
  • be less active
  • take medicines

Some cancers make substances called cytokines. Cytokines are a group of proteins in the body that play an important part in boosting the immune system. These can cause fatigue.

Some cancers make toxic substances that stop cells making chemicals in the body. Some of these chemicals include potassium or calcium. They’re important for keeping your muscles and heart working. You might feel sleepy and fatigued if their levels are low.

Cancer in your lungs can cause breathlessness and this can make you feel fatigued. A build up of fluid in your tummy (ascites) can also make you feel tired. This is because you’re not able to breathe properly.

Low levels of red blood cells (anaemia)

Cancer and its treatment can affect your bone marrow. The bone marrow is where your body makes red blood cells which carry oxygen around your body.

A lower than normal red blood cell count is called anaemia. Having too few red blood cells means your blood carries less oxygen and you can have:

  • shortness of breath
  • tiredness and lack of energy
  • dizziness
  • difficulty concentrating
  • an increase in heart rate
  • chest pain
  • depression
Let your doctor know if you have any of these symptoms. A blood test can easily check your red cell count and show if you’re anaemic. You might need to have a blood transfusion if your red cell count is very low.


Cancer can sometimes cause pain. Dealing with pain can make you feel very tired. So controlling pain can help to reduce fatigue. Strong pain killers such as morphine can also cause fatigue.

How you feel emotionally

Being diagnosed with cancer can be hard to accept. You’re likely to go through a range of emotions before, during and after your treatment. This is very normal.

You might have a lot of worries, some of these might include:

  • will my treatment work?
  • will I be able to deal with side effects?
  • how will my family and friends cope?
  • will I have enough support?
  • will I be able to keep working?
  • how will I get to the hospital for my treatment?
  • will the treatment be painful?
  • what if I lose my hair?

All these worries can make you feel anxious or down. Anxiety and depression are common in people with cancer. They’re often very draining emotions.

Some people get depressed after they’re diagnosed with cancer. Depression is a medical illness. It’s like having a broken leg or a heart condition. It needs treatment. People with depression often feel as if they have no energy at all. You might have:

  • to drag yourself out of bed in the morning
  • trouble sleeping even though you're exhausted
  • trouble doing daily chores
  • problems concentrating
  • changes to your appetite - you may lose weight
  • trouble remembering things
Tell your doctor or nurse if you think you are depressed. You might be able to have counselling or medicine such as anti depressants to help you cope.

Side effects of treatment 

Fatigue is a common side effect of many cancer treatments. Even though you might not be able to stop your treatment, knowing it's the cause of your fatigue can help you to cope better.

The side effects of cancer treatments can also make fatigue worse. Feeling sick, having trouble sleeping or generally feeling low can make you feel like you have less energy. The following treatments can cause fatigue:

You’re likely to feel some fatigue after surgery for your cancer. This usually gets better with time. This may last for a few weeks or months after your operation depending on what you’ve had done.

People often underestimate how long it takes to get over surgery. Surgery stresses your body and it needs time to heal. You may have felt quite anxious before your operation. And the feeling of letting go and relaxing afterwards can leave you very tired.

Other things that can add to feeling fatigued after surgery are:

  • pain
  • the anaesthetic
  • other medicines such as pain killers like morphine
  • having other treatment soon after surgery such as chemotherapy or radiotherapy 

Most people having radiotherapy feel increasingly fatigued as they go through their treatment. Travelling back and forth to the hospital for treatment can add to this and make you feel very tired too.

There is also a direct effect from the radiation itself. This is because your body is using energy to repair damage to healthy cells from the radiation. For some people, fatigue can last for several weeks or even months after treatment has finished. 

Some people who have radiotherapy to the brain can get an extreme form of tiredness called somnolence syndrome. This is more common in children but can also happen in adults.

Other problems or factors increasing fatigue during or after radiotherapy treatment include:

  • problems sleeping
  • depression
  • having advanced cancer
  • working while having treatment
  • caring for children at home 

Chemotherapy circulates throughout your body in the bloodstream. So it can treat cancer cells almost anywhere in the body. This is known as systemic treatment. But it can also affect some healthy cells.

Nearly everyone who has chemotherapy has some fatigue. This is because most chemotherapy drugs affect your red blood cell count. It can cause the levels to drop between treatments, this is called anaemia. Many people feel very fatigued during this time. It is also because the body is continually using energy to repair itself. 

Depending on the type of chemotherapy you had, your levels usually start to go back up again within 2 weeks. You start to feel less tired and get some energy back. You generally start to feel at your best before your next treatment. The treatment cycle allows you to recover just enough before you need your next cycle. You have regular blood tests to check the levels of red cells in your blood.

Knowing that you will get tired again can make you feel anxious and frustrated. But you’ll get to know what you can and can’t do.

Feeling or being sick, diarrhoea, not drinking enough and other medications can add to your fatigue. It’s important to look after yourself and let your doctor or nurse know if you have any side effects. They can usually help treat them or find ways to help you cope.

Targeted cancer drugs work by targeting the differences in cancer cells that help them to grow and survive.

Immunotherapy uses our immune system to recognise and attack cancer cells. This can cause a number of side effects during or some months after treatment has finished. Rarely some side effects can be life threatening. Side effects can include:

  • high temperature (fever)
  • chills
  • muscle aches
  • headache
  • general tiredness

Doctors don’t yet fully understand how these new treatments cause fatigue. You might get fatigue from these drugs because of the side effects they cause. Or it could be because of how these drugs work in the cells of our body.   

Hormone treatment can cause fatigue as it blocks or lowers the amount of hormones in the body. You might have hormone treatment if you have breast or prostate cancer for example.

After surgery for thyroid cancer, you usually take thyroid hormones to replace hormones. These hormones can cause weakness and difficulty in sleeping. Sometimes this can lead to feeling fatigued.

Other drugs that can also make you feel drowsy or tired include:

  • painkillers
  • anti sickness drugs
  • anti depressants
  • cough medicines
  • sleeping tablets
  • steroids

Fatigue can be worse if you are taking a combination of these drugs.

Speak to your doctor if you think your medicines are making you unnecessarily tired.

What else can cause fatigue

Many other factors can make you feel tired and fatigued if you have cancer. Some of these include:

  • not sleeping well at night or sleeping too much during the day
  • treatment may be harder for you to cope with especially if you're elderly
  • your tiredness may make it harder for you to concentrate so everything seems more difficult making you feel even more tired
  • travelling to and from the hospital for treatment 
  • having a lot of visitors when you are staying in hospital
  • looking after children
  • other health problems such as diabetes, problems with your lungs, heart problems and being overweight

You can ask your nurses to tell your visitors that they can only stay with you for a short time. Don't feel bad if you have to do this. You need a lot of rest and your friends and family will understand.

Causes of long term fatigue

Fatigue for people having treatment for cancer is different from the fatigue some people feel long after finishing their treatment. Or those living with cancer. This is also called chronic fatigue. The symptoms last for at least 6 months or more.

Things that can cause long term fatigue include:

  • bone marrow transplants
  • cancer treatment for brain tumours as well as the cancer itself
  • taking the drug tamoxifen for several years
  • radiotherapy treatment for brain cancer
Last reviewed: 
03 Jan 2020
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  • The information on this page is based on literature searches and specialist checking. We used many references and there are too many to list here. Please contact with details of the particular issue you are interested in if you need additional references for this information.

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