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. Sleep and rest don't fully relieve it, 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. It can also raise levels of natural substances called cytokines, which cause fatigue. Treatments can lower your energy too. Those include surgery, chemotherapy, radiation therapy, hormone therapy, immunotherapy, and targeted therapy. Having more than one treatment can add to fatigue. So can other side effects, such as anemia, pain, infection, poor sleep, and appetite loss. The stress of cancer adds to it as well.

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.