Carcinogen is any substance that can lead to cancer in the body. Many such substances are known such as tobacco, alcohol etc. Many chemicals and drugs and radiation are also carcinogens. While they do treat cancers and cure them, they are themselves are capable of causing cancers. But in very small number of patients. Nowadays this fact is kept in mind all the time. Hence the doses, drugs and schedules are designed keeping them in mind to have the least incidence of second cancers due o the drugs and radiation.