Nicotine isn't just 'extra effective caffeine.'

The research seems to indicate it's a cancer accelerant -- it doesn't cause cancer directly, but it promotes the growth and spread of existing tumors.

Studies suggest it does this through several pathways including:

(a) preventing cancer cells from self-destructing (b) making tumor cells divide more quickly (c) stimulating the growth of new blood vessels in and around tumors, helping them grow; and (d) helping cancer cells metastasize more easily by increasing mobility via epithelial-mesenchymal transition.

It also lowers the effectiveness of chemo and radiotherapy.

Admittedly, most of these are animal/lab studies, in part because it's hard to design a good study in humans since so many patients also use/used cigarettes and vapes, which confounds the results. Still, there's enough evidence out there to be cautious.