What causes fatigue?

The cause of cancer-related fatigue (CRF) is not fully understood. There may be many reasons for it. These include:

  • the cancer itself
  • cancer treatments
  • anaemia
  • eating problems
  • other health problems
  • psychological effects of cancer.

You may feel tired before treatment starts because of the tests and scans you have had to diagnose the cancer. If you are older, have other health problems, or have more than one type of treatment, you are more likely to be affected by cancer-related fatigue.

The cancer itself

For some people, the cancer itself may cause fatigue.

This might be because of the symptoms of cancer. For example, parts of your body may be swollen because the cancer has caused a build-up of fluid. This can make them feel heavy and it can be difficult to move around.

You may feel tired because the cancer has reduced the number of red blood cells. Red blood cells are made in your bone marrow. They contain haemoglobin which carries oxygen to cells. If the haemoglobin level goes down and cells are not getting enough oxygen, you are likely to feel very tired. A reduced number of red blood cells is called anaemia (see below).

Cancers that cause changes in hormone levels, such as breast or prostate cancer, may cause fatigue.

Cancer may also affect the levels of cytokines in the body. Cytokines are a type of protein made in the body. They help control some of the things that cells do. Studies show that cytokine levels can be higher in people with cancer-related fatigue than in people without it. This may cause some symptoms such as tiredness. But we still do not fully understand whether cytokines cause tiredness.

I think people find fatigue difficult to understand. If I tell someone I’m in pain they accept it, but if say I’m tired they think, “So what? I’m tired too”.


Cancer treatments and fatigue

Apart from treatment-related anaemia, doctors are still trying to find out exactly why cancer treatments cause fatigue. It is thought that fatigue may happen after having cancer treatment because:

  • the body needs extra energy to repair and heal
  • there is a build-up of chemicals as the cancer cells are destroyed
  • the body’s immune system is affected.


Many people feel tired after surgery and need to avoid doing too much for a while. This effect does not usually last long. However, some types of surgery may cause continuing problems with fatigue. For example, surgery to the stomach may cause problems with absorbing food. Not being able to absorb nutrients from food can affect your energy levels.

Chemotherapy and radiotherapy

Chemotherapy is the use of anti-cancer (cytotoxic) drugs to destroy cancer cells. It is often given in cycles a few weeks apart. Some people feel most fatigued in the first few days after chemotherapy. They then find it gets better for a while before the next cycle. The fatigue may increase with each treatment.

Radiotherapy is the use of high-energy rays to destroy cancer cells. Often, people feel more fatigued as the treatment goes on.

Fatigue caused by chemotherapy or radiotherapy usually improves after treatment, but sometimes it can become a long-term problem. Sometimes these treatments may cause long term effects such as breathlessness or heart problems. These are likely to make you feel more tired.

Many people find their normal levels of energy return within 6 to 12 months of the treatment finishing. However, some people find they still feel tired and have low energy levels a year or so later. Sometimes, tiredness can continue for 2 years or more, although this is much less common.

Hormonal therapies

Hormonal therapies can stop or slow down the growth of some cancer cells. Some types of hormone therapy reduce the levels of particular hormones in the body. Other types prevent the hormones being absorbed by cancer cells. Hormonal therapy is often given for several years. Some hormonal therapies can cause fatigue.

Targeted (biological) therapies

Targeted therapy uses substances that target the growth of cancer cells. Some targeted therapies can cause fatigue.


Anaemia is a possible cause of cancer-related fatigue.

Anaemia is caused by not having enough haemoglobin (Hb) in the blood. Haemoglobin is found in red blood cells. It carries oxygen around the body. As red blood cells move around the body, they carry oxygen to all the body’s cells. This gives the body energy.

If the number of red blood cells is low, there is less haemoglobin. This means less oxygen reaches the cells. If the level of haemoglobin in the blood drops below normal, you may feel tired and have less energy.

Your doctors will regularly check your blood cell levels.

If you have anaemia, you may also find that you:

  • are breathless
  • feel dizzy and light-headed.

People with heart problems such as angina may find these problems get worse. Or they may have more chest pain than usual if they are anaemic.

Causes of anaemia

Anaemia can be caused by different things. It may be caused by the cancer itself. This is because some cancers cause fewer red blood cells to be made in the bone marrow. Other cancers may cause bleeding. A loss of blood may lead to anaemia.

Chemotherapy can cause anaemia. It reduces the number of red blood cells you make.

Radiotherapy can also cause anaemia if it is given to an area of the body that contains bone marrow. Bone marrow is found inside our bones, mainly in the hip bone and the breast bone (sternum). It is where red blood cells are made. If you have radiotherapy for breast cancer, for example, the radiotherapy will affect the breast bone.

If you are anaemic, your doctor may be able to give you treatment that can help. The treatment for anaemia depends on the cause. The main treatment is a blood transfusion. This is a drip (transfusion) of red blood cells given directly into the bloodstream. This can quickly increase the number of red blood cells in the body.

If you are having chemotherapy, you may be offered a drug called erythropoietin, which can help increase the number of red blood cells in the body.

Eating problems

Our bodies get energy from the food we eat. Fatigue can happen if the body does not get enough food, or if there are changes to the way the body is able to use the food. If you have cancer, this can happen because:

  • you cannot eat the same amount of food as you normally would
  • your body needs more energy than it did before
  • your body may not be able to absorb and use all the nutrients from the food.

You may lose weight even if you are eating a lot, because of the effect of the cancer on the body.

If you feel sick (nausea), you may eat less. This could mean that you do not get enough energy from food. If you are sick (vomit), your body does not absorb the food and its nutrients. This can make you feel weak and tired, and you may also become dehydrated.

If you have nausea or vomiting, your doctor can prescribe anti-sickness (anti-emetic) drugs, which usually help. Some anti-sickness drugs can cause tiredness and may make you feel drowsy. However, it is important to keep taking them. Let your doctor know if this is a problem.

If you cannot take anti-sickness tablets or keep them down due to vomiting, speak to your doctor or nurse. There are other ways of taking anti-sickness medicines. Your healthcare team can discuss this with you.

Chemotherapy can cause changes in appetite and taste, which may cause you to eat less. There are things you can try that may help with this. Your doctor, nurse or hospital dietitian may be able to help.

You may find that you use all your energy cooking and then feel too tired to eat. It can help to ask someone else to prepare food for you, or buy some ready-made meals. Some organisations deliver ready-made meals to your home. You can also contact your council’s social services department to find out if you qualify for their ‘meals on wheels’ service.

You could try having regular, small amounts of food or snacks, rather than a big meal. If you do not feel like eating, you could try ready-made, high-calorie drinks. These are available from any chemist. Some are available on prescription. Unflavoured high-energy powders can add calories to food without making your portion any bigger. These are also available on prescription.

We have more information about eating problems and coping with eating difficulties caused by cancer or its treatment.

Pain and other cancer symptoms

Many people with cancer do not have pain. But pain can cause fatigue. The best way to deal with fatigue caused by pain is to manage the pain.

If you are in pain, talk your GP, cancer doctor or specialist nurse about getting some painkillers to help with this. Some painkillers can make you feel drowsy or sleepy.

Some people find that complementary therapies such as acupuncture and relaxation help their pain.

Other medical problems

Other health conditions like diabetes, heart problems or low thyroid function can also cause fatigue. Having a cancer diagnosis and other health conditions may make the fatigue worse.

Some medicines for other health conditions can also make you feel tired.

Psychological effects of cancer

Fatigue can be made worse by:

  • anxiety
  • depression
  • stress and tension
  • a poor sleeping pattern.

It is common for people to have anxiety or depression when they are first diagnosed with cancer. However, these feelings generally get easier to manage as you come to terms with what has happened.

You may find it helpful to discuss how you feel with your partner, a family member or close friend. Some people find it helpful to talk to other people at a local support group, or join an internet support group.

Macmillan’s Online Community is a place you can talk to others who understand what you are going through. Visit community.macmillan.org.uk

If you find that your mood is low most of the time, you may have depression. If you have depression, your GP will discuss possible treatments with you. They can refer you to a counsellor and prescribe medicines to help if necessary.

Once I’ve pulled myself off the mattress in the morning, the exercise I do actually makes me feel prepared to take on whatever life throws at me that day.


Loss of muscle strength (deconditioning)

If you have fatigue, you may become less active over time. When your muscles are not being used regularly, they become weaker. This is called deconditioning. Having weak muscles means you will find it very difficult and tiring to do even simple tasks. It may also mean you are more at risk of falling over.

It is understandable that you may not want to do very much when you are dealing with cancer and its treatment. But staying active is the best way to keep your muscle strength and build up your energy.

You could start by sitting up in a chair rather than lying down in bed while you are recovering. Gradually build up to walking short distances around the house. Talk to your cancer doctor, specialist nurse or GP about being referred to a physiotherapist. They can help you with an exercise programme to build muscle strength safely.

We have more information on physical activity and cancer.

After every meal I took Laurel and Hardy (my two chemo pumps) for a walk around the ward. It turned out that I was doing about 2 miles a day.


Back to Tiredness (fatigue)

What is fatigue?

Fatigue is feeling very tired most, or all, of the time. It is a very common problem for people with cancer.

Living with fatigue

Planning ahead can help if you have fatigue. There are things you can do to make everyday life easier.