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).
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.