Can chemo cause psychosis? My mother in law finished chemo and radiation after being diagnosed with breast cancer. She was doing well. Driving and living alone. She has no history of psychiatric disorder. She started not feeling well in April ( headache and nausea ) and has deteriorated rapidly since then. She is in a geriatric psych center in psychosis. Refusing all treatment, combative, will not walk or eat, lost 25 pounds since April. All test results are negative and the Dr s do not know what caused the psychosis.