What causes fatigue?

Cancer-related-fatigue (CRF) may be caused by many things. These include:

  • The cancer itself.
  • Treatments for cancer – including chemotherapy, radiotherapy, hormonal therapy, targeted therapies and surgery. CRF usually improves when you have finished treatment but can sometimes be a long-term problem.
  • Low levels of red blood cells (anaemia) due to your cancer or cancer treatments. You may have a blood transfusion to help increase the number of red blood cells.
  • Poor appetite due to your cancer or side effects of treatment.
  • Symptoms that cancer may cause such as pain, breathlessness or fluid retention.

Cancer treatments and fatigue

Before treatment starts, many people already feel tired from the tests and investigations used to diagnose the cancer. The cancer itself can also make people feel tired. They may also be tired from trying to cope with different emotional effects. People who are older, have other medical problems, or have more than one type of treatment are more likely to be affected by cancer related fatigue.


Many people feel tired after surgery and need to take things easy for a while. This effect is usually temporary. However, some types of surgery may cause continuing problems with fatigue. For example, surgery to the stomach may lead to problems with absorbing food. If you are unable to absorb nutrients from food this 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 and then find it gets better until the next cycle. The fatigue may increase with each round of treatment.

Radiotherapy treats cancer by using high-energy rays to destroy cancer cells. Often people may feel increasingly fatigued as the treatment goes on.

Fatigue caused by chemotherapy or radiotherapy usually improves after treatment, but sometimes it can be more of a long term problem.

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

Hormonal therapies

Hormonal therapies are treatments that can stop or slow the growth of some cancer cells. They either alter the levels of particular hormones in the body or prevent the hormones from being absorbed by cancer cells. These are often given for several years. Some hormonal therapies can cause fatigue.

Targeted (biological) therapies

Targeted therapies use substances that target the growth of cancer cells. Some of them can cause fatigue.

Apart from treatment related anaemia, doctors are still trying to find out exactly why cancer treatments cause fatigue. It’s thought that fatigue may occur 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.

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Anaemia and fatigue

Anaemia is a possible cause of fatigue in people with cancer.

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 circulate in the body, they give energy by carrying oxygen to all of the body’s cells.

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

Doctors regularly check the levels of blood cells in people who have cancer and cancer treatments.

People who have anaemia may also find they:

  • are breathless 
  • feel dizzy and light-headed
  • have a worsening of angina (chest pain due to heart problems).

Causes of anaemia

Chemotherapy reduces the production of red blood cells and is a common cause of anaemia.

Radiotherapy can also cause anaemia if it’s given to an area of the body that contains bone marrow. Red blood cells are made in the bone marrow. Radiotherapy given to the breastbone (sternum), the hip bones, or the long bones of the arms and legs is most likely to reduce the production of red blood cells.

If you are having cancer treatment, you may find it helpful to write down your Hb levels in a fatigue diary. This may help you see how your Hb levels affect your everyday life and your level of fatigue. If you are anaemic, your doctor may be able to give you treatment that will make you feel better.

The treatment given for anaemia depends on the cause.

The main treatment is a blood transfusion, which involves a drip (transfusion) of red blood cells given directly into the bloodstream. It can quickly raise the number of red blood cells that circulate in the body.

Eating problems

Our bodies get energy from the food we eat. Fatigue can occur if the body doesn’t get enough food or if there are changes to the way the body is able to use the food. In cancer, this can happen because:

  • you can’t 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.

Sometimes people may lose weight, regardless of the amount they eat, because of the effect of the cancer on the body.

If you feel sick (nausea), you may not get enough energy from food because it’s likely that you are eating less. If you actually are sick (vomit), your body doesn’t absorb the food and essential nutrients it needs. 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. Try to take them regularly so that the sickness doesn’t come back. 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. If you find that some foods no longer appeal to you, try something different. Your doctor, nurse or hospital dietitian may be able to help.

It can help to get someone else to prepare food for you. Otherwise, you may find that you use all your energy to cook and then feel too tired to eat. You could also buy some ready-made meals or place an order with an organisation that delivers 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 small amounts of food or snacks often rather than a big meal. If you don’t 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, which add calories to food without adding bulk, are also available on prescription.

Our section on eating problems has more information on coping with eating difficulties caused by cancer or its treatment.

Pain and other cancer symptoms

Many people with cancer don’t have pain, but for those who do, it can cause fatigue.

Painkillers and other treatments, such as relaxation and acupuncture, can help relieve pain and reduce fatigue.

Other symptoms such as breathlessness (shortness of breath) or fluid retention are also common causes of fatigue. If you have an infection or fever (high temperature), your body needs more energy and this may lead to fatigue. Treating the different symptoms that are causing your fatigue can often help to relieve it, so let your doctor or nurse know about any symptoms you have.


Cytokines are proteins produced by the body. They act as chemical messengers and help regulate a wide variety of functions in the body. Studies have shown that cytokine levels are often raised in people with cancer-related fatigue, and these high levels may actually cause some of the symptoms people have. However, the exact way cytokines cause fatigue is not yet fully understood.

Other medical problems

Other medical problems, for example diabetes, heart problems or low thyroid function may also make the symptoms of fatigue worse. These medical conditions may not have been obvious before the cancer was diagnosed, but they may come to light during or after cancer treatment.

Some medicines for conditions other than the cancer can also make you feel tired.

Back to Tiredness (fatigue)

What is fatigue?

Fatigue is feeling very tired most, or all, of the time. It can sometimes be caused by cancer or cancer treatment.

Tips for better rest

Tiredness can affect your sleeping patterns. There are ways to manage this so you get the most out of your rest.

Managing your fatigue

Making some simple changes to your diet and exercise routine may help manage symptoms of fatigue.