As is obvious from their origins, the above cancer chemotherapies are essentially poisons. Patients receiving these agents experienced severe side-effects that limited the doses which could be administered, and hence limited the beneficial effects. Clinical investigators realized that the ability to manage these toxicities was crucial to the success of cancer chemotherapy.
Several examples are noteworthy. Many chemotherapeutic agents cause profound suppression of the bone marrow. This is reversible, but takes time to recover. Support with platelet and red-cell transfusions as well as broad-spectrum antibiotics in case of infection during this period is crucial to allow the patient to recover.
Several practical factors are also worth mentioning. Most of these agents caused very severe nausea (termed chemotherapy-induced nausea and vomiting (CINV) in the literature) which, while not directly causing patient deaths, was unbearable at higher doses. The development of new drugs to prevent nausea (the prototype of which was ondansetron) was of great practical use, as was the design of indwelling intravenous catheters (e.g. Hickman lines and PICC lines) which allowed safe administration of chemotherapy as well as supportive therapy.