What Is Chemotherapy — and How It Works
Most of what people know about chemotherapy comes from a relative who had it years ago, or a scene in a film. Understanding the mechanism will not make it easy — but it will make every side effect make sense rather than feel like a shock.
Medically reviewed by Dr. Naresh Gundu, Medical Oncologist, MBBS (Chalmeda Anand Rao Institute of Medical Sciences, Karimnagar) · DNB Internal Medicine (Sir Gangaram Hospital, New Delhi) · DM Medical Oncology (AIIMS) · Last reviewed September 2026
- Targets fast-dividing cells — Cancer cells divide more rapidly than most normal cells, which is what chemotherapy exploits.
- Some healthy cells are caught too — A few normal tissues — hair follicles, gut lining, bone marrow — also divide quickly, which explains the most common side effects.
- Given in cycles — The gaps between treatment rounds are not wasted time. They give fast-dividing healthy cells a chance to recover.
- Different classes for different cancers — Your oncologist chooses based on your cancer type, stage and overall health. There is no single chemotherapy.
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Chemotherapy is a class of medicines that targets cells as they divide. Cancer cells divide far more often than most normal cells, so they are more exposed to the medicine and more likely to be damaged by it. That same mechanism explains why some healthy tissues — hair follicles, gut lining, bone marrow — are also affected.
What is chemotherapy and how does it work?
Chemotherapy works by disrupting the process cells use to divide and make copies of themselves. Cancer cells divide far more often than most normal cells, so they encounter the medicine more frequently — and are more likely to be damaged by it.
The medicine targets cell division itself, not cancer cells specifically. This is why it works on cancer, and also why some normal tissues are affected alongside it.
That second part — healthy cells catching some of the impact — is the source of nearly every common side effect. Understanding which normal cells divide quickly, and which do not, makes those effects predictable rather than mysterious.
Why are some healthy cells affected too?
| Cell type | How often it divides | What this can mean during treatment |
|---|---|---|
| Cancer cells | Very rapidly | The primary target of chemotherapy |
| Bone marrow cells | Rapidly | Blood counts can fall — monitored by regular blood tests |
| Gut lining cells | Rapidly | Can cause nausea, mouth soreness, or loose stools |
| Hair follicle cells | Rapidly | Can cause hair thinning or loss |
| Heart muscle cells | Very slowly | Rarely affected by most chemotherapy classes |
| Most nerve cells | Very slowly | Less commonly a direct target |
Where to find us
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MBBS, MD(General Medicine), DM(Medical Oncology)(Adyar,Chennai), ECMO, MRCP SCE(UK)
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MBBS, MD (General Medicine), DrNB (Medical Oncology), ECMO, MRCP SCE (Medical Oncology) (UK)
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MBBS (AIIMS), MS (Surgery) (AIIMS), DNB (Surgical Oncology), MRCS (Edinburgh)
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What should you sort out before your first cycle?
- Tell your team every medicine, supplement and herbal remedy you are taking — some interact with how chemotherapy behaves in the body
- Ask which side effects are typical for your specific treatment and which would need a same-day call
- Confirm who to contact after hours if a symptom worries you
- Ask when your first blood test is and what it will be checking for
- Arrange for someone to be with you on treatment days, especially the first one
- Ask whether other health conditions you have — diabetes, heart disease, kidney problems — will change how you are monitored
What do these words actually mean?
- Cycle
- One complete round of treatment — the days you receive chemotherapy plus the rest days that follow before the next round begins.
- Infusion
- Chemotherapy delivered into a vein through a drip, usually at a day care centre, over a period of minutes to several hours.
- Complete blood count (CBC)
- A blood test measuring your red cells, white cells and platelets. Run before each cycle to check it is safe to proceed.
- Nadir
- The point in your cycle when blood counts are at their lowest. Your team will tell you when yours typically falls so you know when to be most careful about infection and fatigue.
- Response
- How well your cancer is reacting to treatment. Assessed by scans or markers specific to your cancer type, not by how you feel from day to day.
What do people most want to know before they start?
Does the infusion hurt?
The drip itself is not painful for most people. Some notice a cold sensation or mild pressure at the site when it starts. Any pain or burning at the infusion site should be told to your nurse immediately — it usually means the needle needs repositioning. Discomfort in the hours and days after, from nausea or fatigue, is more common than pain during the infusion itself.
Will I lose my hair?
Whether hair is affected depends on the class of chemotherapy, not chemotherapy in general. Your oncologist will tell you what to expect before you start. Hair that falls during treatment almost always regrows after it ends, though the texture may differ for the first few months. Scalp cooling during infusion can reduce hair loss for some people and some treatment classes — ask your team whether it is relevant to yours.
How will I know if it is working?
You will not be able to tell from how you feel day to day. Feeling well does not mean treatment is failing, and feeling unwell does not mean it is working. Response is assessed by scans — usually after a set number of cycles your oncologist will name in advance — and sometimes by blood markers specific to your cancer. Ask when the first assessment is planned so you are not guessing between appointments.
Can I keep eating normally?
Most people can eat, though nausea and taste changes can make the few days closest to each treatment harder. Small, frequent meals tend to be easier than large ones. If you are taking herbal tonics, protein supplements or ayurvedic preparations, tell your team — some change how chemotherapy is processed in the body, not by cancelling it out but by altering its behaviour. Asking is not a judgement; it keeps your treatment safe.
Can I still go out and be around people?
Between cycles, most people are well enough to go out and live fairly normally. The nadir — when blood counts are at their lowest — is when your team may advise more caution around crowds or anyone who is unwell. Ask your oncologist when your nadir is likely to fall so you can plan around it rather than staying home throughout.
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Frequently asked questions
Is chemotherapy the same as poison?
Chemotherapy medicines are toxic to rapidly dividing cells — that is how they work. Unlike a poison, which causes indiscriminate harm, chemotherapy is dosed for your body weight and kidney function, monitored throughout treatment, and selected for your cancer type. Side effects are a known part of treatment, not accidents. Whether the benefit outweighs those effects is a calculation your oncologist makes based on your specific situation.
Why do some people get very sick from chemotherapy while others seem fine?
Response varies between people and between different classes of chemotherapy. The medicines used, your dose, your kidney and liver function, your age and your individual biology all play a part. The same treatment can cause significant nausea in one person and barely any in another. Your team cannot always predict your response before the first cycle, which is one reason close monitoring in the early cycles matters.
How long does a full course of chemotherapy last?
The length depends on your cancer type, the intent of treatment, and how the tumour responds. Most courses involve multiple cycles spaced two to four weeks apart, with the total number agreed in your treatment plan from the start. Your oncologist will give you a timeline at the beginning. Ask for it in writing so you have something concrete to plan the rest of your life around.
Can chemotherapy affect my ability to have children?
Some chemotherapy classes can affect fertility in both men and women, temporarily or permanently, depending on what is used, the dose and your age. If fertility matters to you, this conversation needs to happen before treatment starts — not after. Most centres can arrange a fertility referral quickly when time is short. Do not assume it is not relevant to you; ask your oncologist directly.
What does it mean that chemotherapy is given as day care?
Day care means you come to the treatment centre, receive your chemotherapy through a drip, and go home the same day — usually within a few hours. You do not need to stay overnight unless something unexpected happens. CION administers chemotherapy as day care across its centres. Bring someone with you for your first session, and your team will tell you what to watch for once you are home.
Will chemotherapy affect my memory and concentration?
Some people notice difficulty with memory, focus or word-finding during or after treatment — sometimes called 'chemo brain'. It is a recognised effect, though not universal, and for most people it improves after treatment ends. If you notice it, tell your team; it is worth documenting. It is not a sign of permanent damage, and knowing about it means your team can account for it.
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