What Is Chemotherapy?
A plain-language explanation of how chemotherapy works, why it causes side effects, and what to expect, based on National Cancer Institute resources.
Source: National Cancer Institute · NCI reviewed 2025-05-15 · Verified 2026-07-02
7 min readBeginnerUpdated 2026-07-02
The 30-second version
Chemotherapy (also called chemo) is a cancer treatment that uses drugs to kill cancer cells. It works by killing or stopping the growth of fast-growing cells. Because it also affects some healthy fast-growing cells, it can cause side effects such as fatigue, but these often get better after treatment ends.
Key takeaways
- Chemotherapy uses drugs to kill cancer cells or stop their growth.
- It can cure cancer, lower the chance it returns, slow its growth, or ease symptoms.
- It is often used together with other treatments like surgery and radiation therapy.
- Side effects happen because chemo also affects healthy fast-growing cells; fatigue is the most common.
- Chemo is often given in cycles, with treatment followed by a rest period.
- Side effects do not tell you whether the treatment is working.
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The full explanation.
The simple version
Chemotherapy (also called chemo) is a type of cancer treatment that uses drugs to kill cancer cells.
It works by killing or stopping the growth of cancer and other fast-growing cells. Doctors use chemotherapy for two main reasons:
- To treat cancer: it can be used to cure cancer, lessen the chance it will return, or stop or slow its growth.
- To ease cancer symptoms: it can shrink tumors that are causing pain and other problems.
Chemotherapy uses drugs to kill or slow the growth of fast-growing cancer cells.
How chemotherapy is used with other treatments
Chemotherapy is used to treat many types of cancer. For some people, it may be the only treatment they receive. But most often, it is used together with other cancer treatments.
When used with other treatments, chemotherapy can:
- make a tumor smaller before surgery or radiation therapy (called neoadjuvant chemotherapy)
- destroy cancer cells that may remain after surgery or radiation therapy (called adjuvant chemotherapy)
- help other treatments work better
- kill cancer cells that have returned or spread to other parts of your body
Why chemotherapy causes side effects
Chemotherapy not only kills fast-growing cancer cells, but also kills or slows the growth of healthy cells that grow and divide quickly. Examples are the cells that line your mouth and intestines and the cells that cause your hair to grow. Damage to these healthy cells may cause side effects, such as mouth sores, nausea, and hair loss.
Side effects often get better or go away after you have finished chemotherapy.
The most common side effect is fatigue, which is feeling exhausted and worn out. You can prepare for fatigue by:
- asking someone to drive you to and from chemotherapy
- planning time to rest on the day of and day after chemotherapy
- asking for help with meals and childcare on the day of and at least one day after chemotherapy
Side effects happen because chemo also affects healthy fast-growing cells.
What to expect
How it is given. Chemotherapy may be given in many ways, including as pills, capsules, or liquids you swallow (oral); directly into a vein through an IV; as an injection; or as a cream you rub onto your skin. It is most often given through an IV, using a thin needle placed in a vein in your hand or lower arm. It may also be given through a catheter (a thin, soft tube), a port (a small disc placed under the skin), or with the help of a pump.
Where you go. You may receive chemotherapy during a hospital stay, at home, or as an outpatient at a doctor's office, clinic, or hospital. Outpatient means you do not stay overnight.
How often. Treatment schedules vary widely. Chemotherapy is often given in cycles. A cycle is a period of treatment followed by a period of rest. For instance, you might receive chemotherapy every day for one week, followed by three weeks with no chemotherapy — these four weeks make up one cycle. The rest period gives your body a chance to recover and build new healthy cells.
It is best not to skip a treatment. Sometimes your doctor may change your schedule if you are having certain side effects. If this happens, your doctor or nurse will explain what to do.
How you will know if it is working
You will see your doctor often. During these visits, they will ask how you feel, do a physical exam, and order medical tests and scans. Tests might include blood tests, and scans might include MRI, CT, or PET scans.
You cannot tell if chemotherapy is working based on its side effects. Some people think that severe side effects mean chemotherapy is working well, or that no side effects mean it is not working. The truth is that side effects have nothing to do with how well chemotherapy is fighting your cancer.
Your doctor uses exams and scans — not your side effects — to see if chemo is working.
Eating and working during chemotherapy
Chemotherapy can damage the healthy cells that line your mouth and intestines and cause eating problems. Tell your doctor or nurse if you have trouble eating. You might also find it helpful to speak with a dietitian.
Many people can work during chemotherapy, as long as they match their work schedule to how they feel. If your job allows, you may want to see if you can work part-time or from home on days you do not feel well. Many employers are required by law to change your work schedule to meet your needs during cancer treatment.
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What Is Chemotherapy: the quick overview
A one-breath explanation you can watch before an appointment.
Coming soonWhat Is Chemotherapy, explained simply
The core ideas with friendly animation and plain language.
Coming soonUnderstanding what is chemotherapy — full lesson
A deeper walkthrough covering the key takeaways and common questions.
Coming soonVideo transcriptâ–¾
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Suggested animation storyboardâ–¾
- 1Open on a calm title card: "What Is Chemotherapy?" with the Cancer Explained mark.
- 2Narrator reads the 30-second summary while a soft animated diagram builds on screen: "Chemotherapy (also called chemo) is a cancer treatment that uses drugs to kill cancer cells. It works by killing or stopping the growth of fast-growing cells. Because it also affects some healthy fast-growing cells, it can cause side effects such as fatigue, but these often get better after treatment ends."
- 3Scene 2: illustrate the idea — "Chemotherapy uses drugs to kill cancer cells or stop their growth."
- 4Scene 3: illustrate the idea — "It can cure cancer, lower the chance it returns, slow its growth, or ease symptoms."
- 5Scene 4: illustrate the idea — "It is often used together with other treatments like surgery and radiation therapy."
- 6Close on a reminder card: this is educational only; talk with your healthcare team, and a link to the NCI source.
Words to know
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Quick knowledge check
According to this article, what does chemotherapy use to treat cancer?
Frequently asked questions
â–¸Does chemotherapy always cause hair loss?
Not always. Hair loss can happen because chemotherapy affects the healthy cells that cause hair to grow, but side effects depend on the type of chemo and how your body responds. Side effects often get better or go away after you finish chemotherapy.
â–¸How is chemotherapy given?
Chemotherapy can be given in many ways, including as pills you swallow, into a vein through an IV, as an injection, or as a cream on the skin. It is most often given through an IV in a vein in your hand or lower arm.
â–¸How long does chemotherapy last?
Treatment schedules vary widely. Chemotherapy is often given in cycles, such as treatment for one week followed by three weeks of rest. How often and how long depends on your cancer, the drugs used, and how your body responds.
â–¸Do bad side effects mean the chemotherapy is working?
No. You cannot tell if chemotherapy is working based on its side effects. Side effects have nothing to do with how well chemotherapy is fighting your cancer. Your doctor uses exams, blood tests, and scans to check how it is working.
â–¸Can I keep working during chemotherapy?
Many people can work during chemotherapy, as long as they match their work schedule to how they feel. You may be able to work part-time or from home on days you do not feel well. Many employers are required by law to adjust your schedule during treatment.
â–¸Why does chemotherapy cause side effects?
Chemotherapy kills fast-growing cancer cells, but it also affects healthy cells that grow and divide quickly, such as those in your mouth, intestines, and hair. Damage to these healthy cells can cause side effects like mouth sores, nausea, and hair loss.
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Questions to ask your healthcare team
Consider bringing these questions to your next appointment.
- What is the goal of my chemotherapy — to cure, control, or ease symptoms?
- How will my chemotherapy be given, and how often?
- How long will my treatment last?
- What side effects should I watch for, and how can I manage them?
- How will you check whether the chemotherapy is working?
- Can I work during treatment, and are there days I should rest?
- Who should I call if I notice signs of infection around my catheter or port?
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