Fatigue occurs in 14% to 96% of people with cancer, especially those receiving treatment for their cancer. Fatigue is complex, and has biological, psychological, and behavioral causes. Fatigue is difficult to describe and people with cancer may express it in different ways, such as saying they feel tired, weak, exhausted, weary, worn-out, heavy, or slow. Health professionals may use terms such as asthenia, fatigue, lassitude, prostration, exercise intolerance, lack of energy, and weakness to describe fatigue.

Fatigue can be described as a condition that causes distress and decreased ability to function due to a lack of energy. Specific symptoms may be physical, psychological, or emotional. To be treated effectively, fatigue related to cancer and cancer treatment needs to be distinguished from other kinds of fatigue.

Fatigue may be acute or chronic. Acute fatigue is normal tiredness with occasional symptoms that begin quickly and last for a short time. Rest may alleviate fatigue and allow a return to a normal level of functioning in a healthy individual. Chronic fatigue syndrome describes prolonged debilitating fatigue that may persist or relapse, and is not related to cancer. Fatigue related to cancer is called chronic because it lasts over a period of time and is not completely relieved by sleep and rest. Chronic fatigue diagnosed in patients with cancer may be called "cancer fatigue", "cancer-related fatigue", or "cancer treatment-related fatigue". Although many treatment- and disease-related factors may cause fatigue, the exact process of fatigue in people with cancer is not known.

Fatigue can become a very important issue in the life of a person with cancer. It may affect how the person feels about him- or herself, his or her daily activities, family care, and relationships with others, and whether he or she continues with cancer treatment. Patients receiving some cancer treatments may miss work or school, withdraw from friends, need more sleep, and, in some cases, may not be able to think clearly or perform any physical activities because of fatigue. Finances can become difficult if people with fatigue need to take disability leave or stop working completely. Job loss may result in the loss of health insurance or the inability to get medical care. Understanding fatigue and its causes is important in determining effective treatment and in helping people with cancer cope with fatigue. Tests that measure the level of fatigue have been developed.

How long fatigue lasts and how much fatigue the patient feels depends on the type and schedule of cancer treatment. For example, patients treated with cycles of chemotherapy usually have the most fatigue in the days following treatment, then less fatigue until the next treatment. Patients treated with external-beam radiation therapy usually have more fatigue as their treatment continues. It is likely that most patients beginning cancer treatment already feel fatigued following diagnostic tests, surgery, and the emotional distress of coping with a cancer diagnosis.

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