chemotherapy, one dose of poison is not enough.
Professor Geoffrey R.Weiss of the University of Texas at San Antonio: 'It has been well documented experimentally and clinically that anti cancer agents do not destroy all cancer cells following each dose' (Clinical Oncology, p.98) So more doses are given,usually at intervals of two to three weeks. As these doses continue, however, cancer cells may 'acquire resistance': 'There is a very narrow margin between doses that produce significant toxicity and those that achieve important cancer-killing result.(p.97).
Significant toxicity does not mean death, but it comes too close for comfort.