If doing a second or third course of chemo for cancer, is the person much more likely to fail treatment?

Not necessarily: Chemotherapy is normally given in multiple courses.

The chemo fails the: Patient, not vice versa! a cycle commonly is the amount of chemotherapy that a person can tolerate due to toxicity. We postulated that each cycle removed a log of tumor cells. Each cancer has its own guideline for number of cycles, regardless, but if the tumor does not respond, it's stopped, and commonly other regimens are used. If 2, 3, or 4 are tried, the tumor is resistant.