Tobacco contains about 60 carcinogens. The vast majority belong to three groups: 1) polycyclic aromatic hydrocarbons, 2) aromatic amines and 3) nitrosamines, the latter related to nicotine. Both tobacco and nicotine weaken the immune system of the individual, leaving him susceptible to various diseases. In addition, both tobacco and nicotine can damage or alter a cell's DNA, which can lead to cancer.
Nicotine is not a substance that directly causes cancer. However, nicotine participates in carcinogenesis by acting as an intermediary of macromolecules by its nitrosation [(bonds between nitrous groups and organic molecules), producing as a result nitrosamines (carcinogenic chemical compounds produced from nitrite and amines)] of tobacco.
