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How chemotherapy works

Chemotherapy works by killing cancer cells and has different effects on different types of cancer. Find out how it works and the different ways you might have chemotherapy.

How chemotherapy kills cancer cells

Chemotherapy circulates throughout your body in the bloodstream. So it can treat cancer cells almost anywhere in the body. This is known as systemic treatment. 

Chemotherapy kills cells that are in the process of splitting into 2 new cells.

Body tissues are made of billions of individual cells. Once we are fully grown, most of the body's cells don't divide and multiply much. They only divide if they need to repair damage.

When cells divide, they split into 2 identical new cells. So where there was 1 cell, there are now 2. Then these divide to make 4, then 8 and so on.

In cancer, the cells keep on dividing until there is a mass of cells. This mass of cells becomes a lump, called a tumour.

Because cancer cells divide much more often than most normal cells, chemotherapy is much more likely to kill them.

Some drugs kill dividing cells by damaging the part of the cell's control centre that makes it divide. Other drugs interrupt the chemical processes involved in cell division.

The effects on dividing cells

Chemotherapy damages cells as they divide.

In the centre of each living cell is a dark blob, called the nucleus. The nucleus is the control centre of the cell. It contains chromosomes, which are made up of genes.

These genes have to be copied exactly each time a cell divides into 2 to make new cells.

Diagram showing how new genes are made for new cells

Chemotherapy damages the genes inside the nucleus of cells.

Some drugs damage cells at the point of splitting. Some damage the cells while they're making copies of all their genes before they split. Chemotherapy is much less likely to damage cells that are at rest, such as most normal cells.

You might have a combination of different chemotherapy drugs. This will include drugs that damage cells at different stages in the process of cell division. This means there's more chance of killing more cells.

Why chemotherapy causes side effects

The fact that chemotherapy drugs kill dividing cells helps to explain why chemotherapy causes side effects. It affects healthy body tissues where the cells are constantly growing and dividing, such as:

  • your hair, which is always growing
  • your bone marrow, which is constantly producing blood cells
  • your skin and the lining of your digestive system, which are constantly renewing themselves

Because these tissues have dividing cells, chemotherapy can damage them. But normal cells can replace or repair the healthy cells that are damaged by chemotherapy. 

So the damage to healthy cells doesn't usually last. Most side effects disappear once your treatment is over. Some side effects such as sickness or diarrhoea might only happen during the days you are actually having the drugs. 

How you have chemotherapy

You can have chemotherapy as:

  • an injection into the bloodstream (usually through a vein)
  • a drip (intravenous infusion) into the bloodstream through a vein
  • tablets
  • capsules

Chemotherapy drugs that you have in these ways circulate all round the body in the bloodstream. They can reach cancer cells almost anywhere in the body. This is known as systemic treatment.

How well chemotherapy works

The chance of the chemotherapy curing your cancer depends on the type of cancer you have.

  • With some types of cancer, most people are cured by chemotherapy
  • With other types of cancer, fewer people are completely cured

Examples of cancers where chemotherapy works very well are testicular cancer and Hodgkin lymphoma.

With some cancers, chemotherapy can't cure the cancer on its own. But it can help in combination with other types of treatment.

For example, many people with breast or bowel cancer have chemotherapy after surgery to help lower the risk of the cancer coming back.

With some cancers, if a cure is unlikely, your doctor may still suggest chemotherapy to:

  • shrink the cancer
  • relieve your symptoms
  • give you a longer life by controlling the cancer or putting it into remission

What remission means

Remission is a word doctors often use when talking about cancer. It means that after treatment there is no sign of the cancer.

You might hear your doctor talk about complete remission and partial remission.

Complete remission

This means that the cancer can't be detected on scans, X-rays, or blood tests, etc. Doctors sometimes call this a complete response. 

Partial remission

This means the treatment has killed some of the cells, but not all. The cancer has shrunk, but can still be seen on scans and doesn't appear to be growing.

The treatment might have stopped the cancer from growing. Or the treatment could have made the cancer smaller so that other treatments are more likely to help, such as surgery or radiotherapy. This is sometimes called a partial response. 

Another term doctors use is stable disease. This can mean that the cancer has stayed the same size or it might even have grown by a small amount.

Last reviewed: 
13 Dec 2014
  • The Chemotherapy Source Book (4th edition)
    Michael C Perry
    Lippincott, Williams and Wilkins, 2008

  • Handbook of Cancer Chemotherapy (8th edition)
    Skeel, R.T. and Khleif, S.N.
    Lippincott, Williams and Wilkins, 2011

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