Allowing time between treatments for the bone marrow to recover is another reason that chemotherapy is given at intervals. In most chemotherapy regimens, more than one drug is used, either simultaneously or sequentially. This is called combination chemotherapy. Different chemotherapy agents stop cell growth at different stages in the cell cycle. By giving a combination of drugs, each of which may interfere with cell growth at different stages of the cell cycle, more cancer cells are likely to be killed.
Different drugs have different side effects.
Because of this, we may try to combine drugs with different side effects to give as much treatment as possible with minimal toxicity. Combination chemotherapy may reduce the chances of the cancer cell becoming resistant to specific chemotherapy agents. Clinical trials have shown that for many types of cancer, combination chemotherapy is more effective than single agent drug therapy. Finally, we give multiple treatments because only a small percentage of cells in a cancerous or malignant tumor or any tissue are dividing at any one time. With repeated treatment, more cancer cells are attacked in the process of dividing.