Why does cancer treatment make you so tired?

Cancer treatment can make you extremely tired because of a condition called cancer fatigue. It's different from ordinary tiredness—it isn't completely relieved by sleep or rest, and it may happen after little or no activity.

Fatigue can come from several sources. The cancer itself uses calories and nutrients your body needs, and can raise levels of natural substances called cytokines that cause fatigue. Treatments—including surgery, chemotherapy, radiation therapy, hormone therapy, immunotherapy, and targeted therapy—can all lower your energy, and having more than one treatment can increase fatigue. Other side effects like anemia, pain, infection, poor sleep, and appetite loss can add to it, as can the emotional stress of cancer.

Cancer fatigue is very common—more than 80% of people with cancer feel it during chemotherapy or radiation therapy. It usually decreases after treatment ends, though for some people it lasts months or years.

Tell your health care team if you feel very weak and tired, especially if rest doesn't help. They can look for causes like anemia or infection and help you find ways to manage it.