If must depend on the particular treatment and circumstance. I was by a loved one's side for the past several months during chemo (stage IV), and we were pretty shocked how few side effects there were. Basically none.

Radiation, on the hand, was brutal.

Cancer can be triggered by a foreign agent, but it itself is not a foreign agent. It is your own body's natural processes gone haywire. That makes differentiating cancer cells from other cells more difficult. Chemotherapy works by essentially killing your most active cells. The cells that are constantly reproducing. Those are your hair follicles, your stomach lining, your cancer, etc. It's why the side effects include hair loss and nausea. It's a chemical shotgun blasted into your body with the hopes that one of the casualties is the cancer.