Nicotine itself may not be a carcinogen, but its metabolites are. Nicotine itself has been demonstrated to be a tumor promoter by way of increasing tumor cell division in lung cancer and inhibiting apoptosis.

Quoting chemical highlight 25-1 from "Organic Chemistry" 6th edition by Vollhardt & Schore:

Nicotine appears to play a dual contributory role, because its metabolites are outright carcinogens and because the parent system itself, while not causing cancer, is a tumor promoter.

The metabolic pathway has as the initial step the N-nitrosation of the azacyclopentane (pyrrolidine) nitrogen. Oxidation and ring opening (compare Chemical Highlight 21-3) then take place, giving a mixture of two N-nitrosodialkanamines (N-nitrosamines), each of which is a known powerful carcinogen.

Upon protonation of the oxygen in the nitroso group, these substances become reactive alkylating agents, capable of transferring methyl groups to nucleophilic sites in biological molecules such as DNA, as shown below. The diazohydroxide that remains decomposes through a diazonium ion to a carbocation, which may inflict additional molecular damage (Section 21-10).

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