Chemotherapy drugs are to cancer cells what antibiotics are to bacteria. They interrupt the cell division process by preventing ongoing duplication of the cellular materials. As the immature cells die, new ones are not able to take their place and tumor growth is stopped. As chemotherapy is delivered in an ongoing fashion, more and more cancer cells are killed off, eventually, hopefully, getting them all.

Chemotherapy schedules are usually spaced at intervals, to give the normal cells a chance to recover. Normal cells are also affected, but only a small fraction of them are in a vulnerable phase at any time. As the chemotherapy is eliminated from the body, the normal cell population recovers.

Chemotherapy delivery has commonly been via an intravenous infusion. This is to ensure reliable delivery to the target. If they are taken in pill form, absorption has to be reliable. Then they have to pass through the liver without being inactivated, before being passed on to the general circulation. Such drugs have been developed and used, but the majority are still via an infusion.