A substance or agent that causes cancer, and which are often (but not necessarily) mutagens or teratogens. They cause cancer by altering DNA. With the exception of tobacco, what is sometimes not recognised by the general public is that almost all carcinogens consumed by humans are contained in plants, something that prevents plants from being eaten by animals. Bracken, for example, contains large amounts of carcinogens.
See DNA (deoxyribonucleic acid), Environmental tobacco smoke (ETS), Mutagens, Teratogen