Carcinogen is a substance capable of causing cancer in living tisue. Some car inogens do not affect DNA directly, but lead to cancer in other ways. For example, they may cause cells to divide at a faster than normal rate, which could increase the chances that DNA changes will occur. Carcinogens do not cause cancer in every case, all the time. Carcinogens are not necessarily immediately toxic, thus their effect can be insidious. Cancer is any disease in which normal cells are damaged and do not undergo programmed cell death as fast as they divide via mitosis. Carcinogens may increase the risk of cancer by altering cellular metabolism or damaging DNA directly in cells, which interferes with biological processes. Also, carcinogens can be caused by the use of tobacco.