Normal cells in the body go through a ‘lifecycle’ – that is, they reproduce new cells, age and die. For example, the normal lifecycle of a red blood cell is about 120 days. The reproduced cell is identical it it’s ‘mother’ cell.
Most chemotherapy works on cells that are in the reproduction phase, stopping them from creating new cells. The cancer cells ill then eventually die without creating new cells. This is why you need several rounds of chemo, as some of the cancer cells will not be in the right phase to be targeted the first time However, the drug cannot choose between normal cells and cancerous cells, so it attacks both. This is why chemotherapy has so many unwanted side effects.