The short answer
Anemia means the number of red blood cells is below normal. Red blood cells carry oxygen through your body, so anemia can make you feel very tired, short of breath, or lightheaded. Cancer treatments like chemotherapy and radiation therapy, and cancers that affect the bone marrow, can cause it. Blood tests check for anemia, and there are treatments that may help.
Anemia is a condition in which the number of red blood cells is below normal.
Red blood cells carry oxygen from the lungs throughout your body, so anemia can make you feel very tired.
Signs may include shortness of breath, feeling dizzy or faint, headaches, a fast heartbeat, and pale skin.
Chemotherapy, radiation therapy, and cancers that affect the bone marrow can cause anemia.
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The full explanation.
What anemia is
Anemia is a condition in which the number of red blood cells is below normal. Red blood cells are the cells that carry oxygen from the lungs throughout your body to help it work properly. When you are anemic, your body does not have enough red blood cells to move oxygen the way it should.
Because oxygen reaches your body through red blood cells, having too few can leave you feeling drained.
Signs of anemia
Anemia can make you feel very tired, short of breath, and lightheaded. Signs of anemia may also include:
- feeling dizzy or faint
- headaches
- a fast heartbeat
- pale skin
Tell your doctor if you are extremely tired or notice other signs of anemia. There are treatments that may help.
Why it happens during cancer treatment
Cancer treatments such as chemotherapy and radiation therapy can cause anemia. Cancers that affect the bone marrow—the soft tissue inside bones where blood cells are made—can also cause it.
You will have blood tests to check for anemia. Treatment for anemia is based on your symptoms and on what is causing the anemia. Because there can be different causes, your healthcare team looks at the whole picture before deciding what may help.
Ways to manage anemia
If you have fatigue caused by anemia, these steps may help:
- Save your energy and ask for help. Choose the most important things to do each day. When people offer to help, let them—they can take you to the doctor, make meals, or do other things you are too tired to do.
- Balance rest with activity. Take short naps during the day, but keep in mind that too much bed rest can make you feel weak. You may feel better if you take short walks or exercise a little every day.
- Eat and drink well. Talk with your doctor, nurse, or a registered dietitian to learn what foods and drinks are best for you. You may need to eat foods that are high in protein or iron.
Your healthcare team can tell you whether medicine, iron pills, a blood transfusion, or other treatments might help in your situation. Do not start iron pills or other supplements on your own. Iron only helps if low iron is the cause, and your team needs to know what you are taking.
When to get help sooner
- Call 911 or go to an emergency department if shortness of breath comes on suddenly, or it is bad enough that you struggle to talk, or you pass out.
- Call your doctor immediately if you are having chemotherapy and your temperature reaches 100.4°F (38°C) or higher. That is what CDC tells people on chemotherapy to do, because fever may be the only sign of an infection and an infection during chemotherapy can be life-threatening. Keep the number for out-of-hours as well. If you end up at an emergency department, say you are on chemotherapy as soon as you arrive.
- Call your care team the same day if you have chest discomfort, pain or pressure, a fast or irregular heartbeat, or you feel dizzy or close to fainting when you stand up.
- Call your care team within a day or two if you feel more short of breath than usual with simple activity, if tiredness keeps getting worse, or if your skin looks pale or your eyes look yellow.
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Common questions
What is anemia?
Anemia is a condition in which the number of red blood cells is below normal. Red blood cells carry oxygen from your lungs throughout your body to help it work properly.
What are the signs of anemia?
Anemia can make you feel very tired, short of breath, and lightheaded. Signs may also include feeling dizzy or faint, headaches, a fast heartbeat, and pale skin. Tell your doctor if you notice these signs.
Why does cancer treatment cause anemia?
Cancer treatments such as chemotherapy and radiation therapy, as well as cancers that affect the bone marrow, can cause anemia. When you are anemic, your body does not have enough red blood cells.
How is anemia found and treated?
You will have blood tests to check for anemia. Treatment is based on your symptoms and on what is causing the anemia. Your healthcare team can explain the options that fit your situation.
What can I do to feel better if I have anemia?
NCI suggests saving your energy and asking for help, balancing rest with activity, and eating and drinking well. Talking with your doctor, nurse, or a registered dietitian can help you learn what is best for you.
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Last updated: 2026-08-18Next planned review: 2027-01-02
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How this page was created
Cancer Explained does not originate medical claims. Every page restates guidance already published by the National Cancer Institute, the CDC, the USPSTF and the FDA, in plain language, with the source cited so you can check the original yourself. AI does the translating and organizing; automated checks test claims, citations, clarity and safety before anything publishes. We do not employ clinicians and do not intend to — our work is translation and navigation, not clinical judgment. Nothing here is personal medical advice, and no page can account for your particular situation.
Editorial status: Source checked — This page was written with AI assistance and checked line by line against the sources listed on it. That confirms the sources support what the page says. It is not a medical review, and it does not confirm the page is complete or right for your situation.
Human medical review: not completed. Pages here are not signed off by a clinician before they publish. That is not an oversight we are quietly working around: we restate published guidance and cite it, so the authority belongs to the source rather than to us, and every page names where its claims come from — you can verify us instead of trusting us. Where a volunteer clinician has reviewed a page, their name and credentials appear on it; where no name appears, no clinician has checked it. We are glad to have reviewers and are recruiting them, and we do not hold pages back waiting for one. Use this site to understand your situation and to ask better questions of the people treating you.
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