A carcinogen is any substance, radionuclide, or radiation that is an agent directly involved in causing cancer.This may be due to the ability to damage the genome or to the disruption of cellular metabolic processes.fter the carcinogen enters the body, the body makes an attempt to eliminate it through a process called biotransformation. The purpose of these reactions is to make the carcinogen more water-soluble so that it can be removed from the body.