Chemotherapy is a drug treatment that uses powerful chemicals to kill fast-growing cells in your body. Chemotherapy is most often used to treat cancer, since cancer cells grow and multiply much more quickly than most cells in the body.
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.
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.