Why does anaemia in cancer cause persistent fatigue?

Red blood cells carry oxygen from the lungs to muscles, the brain, and other tissues. Anaemia can develop when cancer affects blood-cell production, causes blood loss, increases inflammation, or when treatment affects the bone marrow. The heart and breathing muscles may work harder to support oxygen delivery. This can leave someone weak, breathless during activity, light-headed, cold, or unable to concentrate. Fatigue may remain after sleep because the underlying oxygen-carrying problem has not been corrected.

Tell your oncology team how fatigue changes your daily activities, walking, sleep, appetite, and ability to work or care for yourself. A blood count can show whether anaemia is present, while other tests may help identify contributing factors. Management depends on the cause and may include treating bleeding or inflammation, adjusting cancer treatment, replacing a documented deficiency, or other clinician-prescribed options. Pace tasks, rest between activities, drink enough fluid unless advised otherwise, and ask for help with strenuous work.

Seek urgent help for breathlessness at rest, chest pain, fainting, new confusion, a racing or irregular heartbeat with weakness, or bleeding that does not stop. Contact your oncology team promptly if fatigue suddenly worsens, you become unable to manage basic activities, or you notice black stools, red urine, or repeated dizziness. These symptoms need assessment rather than self-treatment with iron.

Ask what your blood results show, what may be causing the anaemia, whether bleeding or treatment is contributing, and which activity limits should be reported. Ask before taking iron or vitamins, and clarify when repeat blood testing is planned.