Fatigue is a feeling of being extremely tired, weak, or exhausted. It's the most common side effect of cancer and cancer treatment. But fatigue does not mean the cancer is getting worse or that the cancer treatment isn't working.

Some people with cancer have described fatigue as being tired to the bones, worn out, or hitting a wall. Others say it's the most distressing side effect of cancer treatment. Cancer-related fatigue is different for everyone. It's important that the person who is experiencing it describes how they feel and how it impacts their life. Fatigue may make a person unable to work, do physical activity, be involved with family, socialize with friends, or carry out daily activities. Sometimes it even causes people to miss cancer treatments.

If the person with cancer is easily distracted and unable to concentrate on mental work or activity, then they may have attentional fatigue. This is also sometimes called chemo brain or brain fog. It's a common side effect of cancer treatment.

Depending on its cause, cancer-related fatigue can come and go or stay for a while. Fatigue from chemotherapy tends to be the worst a few days after treatment and then gets better before the next treatment. Fatigue from radiation usually starts slowly a few weeks after treatment starts. It often gets worse as treatment continues. It improves over time after treatment ends. Fatigue from immunotherapy often starts in the first few weeks to months. Depending on the type of immunotherapy, it can happen any time during treatment. Surgery can also cause fatigue, starting right after the procedure and improving as you heal.

We currently understand some of the causes of fatigue, but not all of them. Fatigue may be related to physical changes caused by cancer or its treatment. Cancer-related fatigue tends to be more severe than the fatigue people without cancer people sometimes have. This fatigue lasts longer and does not get better with sleep.

Low hemoglobin levels (anemia) is a common treatment side effect that's also related to fatigue. Hemoglobin carries oxygen in the blood. So if it is low, the body cells do not get as much oxygen as they need. People who are not well-nourished, don't drink enough fluid and are dehydrated, or are not able to move around much tend to have more fatigue. Uncontrolled pain, depression, and problems sleeping can also cause fatigue. Finally, the way a person handles stress, thinks, or behaves can influence fatigue.