Input Junkie

I'd guess that he means nicotine either potentiates carcinogens or exacerbates some aspect of tumor growth (e.g. blood vessel formation, something that without cancer might not have any deleterious effects and even be healing) One of the special things about cancer is that it's a normal process (cell division, the basis of tissue repair) run amok; cancer is a circumstance in which what is normally innocuous or even healthy is potentially lethal, and the deadliest poisons are the preferred medicine.