My understanding is that nicotine itself is not particularly carcinogenic (there are some studies indicating slight increases in tumorigenesis in conjunction with specific carcinogens, and others indicating no increases), but there are a lot of other compounds in tobacco (including chewing tobacco) that are highly carcinogenic. To my knowledge there are no studies of the carcinogenic potential of long-term nicotine replacement therapy in humans.
Nicotine has several metabolites. NNN, NNK and cotinine. The first two are carcinogenic, but the risk is negligible (but not absent) for smoking cessation therapy. The effects of cotinine appear to be slightly negative on brain health, but of course there are dietary and lifestyle factors which play into the cash value of all this.
Nicotine also appears to be slightly deleterious for kidney and heart health, so it's not entirely benign. It also has to be avoided with female oral contraceptives (greater risk with age) otherwise the risk of stroke increases fairly dramatically.
Do I use nicotine knowing these risks? Yes as a cognitive enhancer, but under the assumption that I'll cease before the age of 40 or upon financial success, while in good health and maintaining a fruit and vegetable rich diet.
