The short answer
Radiation can affect nearby healthy cells, which causes side effects. Fatigue is common, and other effects depend on the part of the body being treated. Most healthy cells recover within a few months, but some effects can appear or last long after treatment.
Side effects happen because radiation can affect nearby healthy cells as well as cancer cells.
Fatigue is a common side effect of radiation therapy.
Other side effects depend on which part of the body is treated.
Most healthy cells recover within a few months after treatment ends.
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The full explanation.
The simple version
Radiation therapy is aimed carefully at cancer, but it can still touch some healthy cells nearby. Radiation not only kills or slows the growth of cancer cells, it can also affect these nearby healthy cells — and that is what causes side effects.
Side effects happen because radiation can affect healthy cells near the cancer, not just the cancer itself.
Fatigue is common
Many people who get radiation therapy have fatigue. Fatigue is feeling exhausted and worn out. It can happen all at once or come on slowly.
People feel fatigue in different ways. You may feel more or less fatigue than someone else who is getting the same amount of radiation therapy to the same part of the body.
Other side effects depend on the treated area
Beyond fatigue, the side effects you may have depend on which part of your body is treated. Here are some examples of possible side effects by area:
- Brain: fatigue, hair loss, memory or concentration problems, nausea and vomiting, skin changes, headache, and blurry vision.
- Breast: fatigue, hair loss, skin changes, swelling (edema), and tenderness.
- Chest: fatigue, hair loss, skin changes, trouble swallowing, cough, and shortness of breath.
- Head and neck: fatigue, hair loss, mouth problems, skin changes, taste changes, throat problems such as trouble swallowing, and a less active thyroid gland.
- Pelvis: diarrhea, fatigue, hair loss, nausea and vomiting, sexual and fertility problems, skin changes, and urinary and bladder problems.
- Rectum: diarrhea, fatigue, hair loss, sexual and fertility problems, skin changes, and urinary and bladder problems.
- Stomach and abdomen: diarrhea, fatigue, hair loss, nausea and vomiting, skin changes, and urinary and bladder problems.
It helps to discuss this list with your doctor or nurse and ask about the side effects you might expect.
How long side effects last
Healthy cells that are damaged during radiation treatment usually recover within a few months after treatment is over. As they recover, many side effects get better.
But sometimes people have side effects that do not improve. And other side effects may show up months or years after radiation therapy is over.
Most healthy cells recover within a few months, so many side effects ease after treatment ends.
Understanding late effects
Side effects that show up months or years after treatment are called late effects. Whether you might have late effects, and what they might be, depends on:
- the part of your body that was treated.
- other cancer treatments you have had.
- genetics.
- other factors, such as smoking.
Because late effects vary so much from person to person, it is important to ask your doctor or nurse which ones you should watch for. Keeping up with follow-up visits helps your team spot and manage any problems early.
When to get help sooner
- Call 911 or go to an emergency department if you suddenly cannot catch your breath, or your chest hurts, while your chest is being treated. Go straight there too if your temperature reaches 100.4°F (38°C) or higher and you are having chemotherapy alongside radiation, or if a fever comes with shaking chills or confusion. The CDC describes fever during chemotherapy as a medical emergency.
- Call your care team the same day if your temperature reaches 100.4°F (38°C) or higher on radiation by itself, or the treated skin blisters, weeps, or breaks open. Infection during cancer treatment can be life threatening.
- Call your care team within a day or two if swallowing hurts enough that you are eating less, or loose stools keep coming and you cannot drink enough to keep up. Dehydration from diarrhea is dangerous, so do not wait it out.
Words to know
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Common questions
Why does radiation therapy cause side effects?
Radiation not only kills or slows the growth of cancer cells, it can also affect nearby healthy cells. Damage to these healthy cells is what causes side effects.
Is fatigue common with radiation therapy?
Yes. Many people who get radiation therapy have fatigue, which is feeling exhausted and worn out. It can happen all at once or come on slowly. People feel fatigue differently, and you may feel more or less than someone else getting the same treatment to the same area.
Do side effects depend on where I get radiation?
Yes. Beyond fatigue, the side effects you may have depend on the part of the body that is treated. For example, radiation to the head and neck may cause mouth or throat problems, while radiation to the pelvis may cause diarrhea or bladder changes.
Will my side effects go away?
Healthy cells that are damaged during radiation usually recover within a few months after treatment is over. But sometimes people have side effects that do not improve, and other effects may show up months or years later.
What are late effects?
Late effects are side effects that show up months or years after radiation therapy is over. Whether you might have them, and what they might be, depends on the part of the body treated, other treatments you have had, genetics, and other factors such as smoking. Ask your doctor or nurse which late effects to watch for.
What should I do about my side effects?
Discuss the possible side effects with your doctor or nurse and ask which ones you might expect. They can help you manage them and tell you which late effects to watch for over time.
Questions to ask your doctor
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Sources last checked: 2026-08-13 what this meansLast updated: 2026-08-13Next planned review: 2027-01-21
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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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