A common problem with many cancers is that they become resistant to chemotherapy after one or more courses of treatment. In other words, the chemotherapy works well at first, but future courses do not help as much.
Chemotherapy resistance occurs when cancers that have been responding to a therapy suddenly begin to grow. In other words, the cancer cells are resisting the effects of the chemotherapy. You may hear statements like the "cancer chemotherapy failed." When this occurs, the drugs will need to be changed.