Why does chemotherapy cause nausea and vomiting?

It's a toxin: The brain has an area that triggers nausea and vomiting if something toxic gets in the body. This is what prevented early humans from dying after eating poisonous berries! chemotherapy triggers the same area of the brain, and the body tries to protect itself by vomiting. New anti-nausea drugs are so effective because they target the "vomit center" in the brain.

Chemo nausea: Chemotherapy can cause nausea and vomiting in several ways. Most commonly it affects the central nervous system causing the nausea and vomiting. Some chemotherapies will also affect bowel function.