Tobacco is so definitely harmful I’m not even going to cite a source for that statement. But tobacco contains a lot more chemicals than just nicotine. TobaccoHarmReduction.org says nicotine isn’t harmful, but TobaccoHarmReduction.org also says nicotine isn’t addictive, so I’m not giving them a lot of credit as a source. Wikipedia also says pretty nice things about nicotine, but exactly how nice seems to depend on what day I check. The page appears to be edited a lot, and sometimes claims it isn’t addictive. So I’m going to fact check Wikipedia very carefully here.
Let’s get some basic things out of the way. Nicotine is addictive to at least a certain percentage of the population. You can see that in science or in anyone you’ve ever watched try to quit smoking with and without the aid of nicotine supplements. It is probably true that propensity to nicotine addiction varies in the population, and some people can try it a few times or even use regularly without becoming addicted, the same way opiates are insanely addictive to some people and I actively dislike them. But most people with that propensity won’t ever start using tobacco because smoking is disgusting and if your brain doesn’t make you do it, you won’t.
Nicotine imitates choline in some but not all choline receptors (conveniently, the ones it affects are called nicotinic receptors). Neurotransmitters are weird and powerful and we don’t fully understand them. At best, nicotine could be in the same category as medications meant to treat depression, which have some costs and some benefits, all of which vary widely between individuals, and whether a given person is better off with or without it depends heavily on their exact mix of genetics and current environment.
Nicotine is poisonous in large enough quantities. This is not necessarily damning: lots of things are poisonous in large doses including vitamin A, which has vitamin right in the name.
More to the point, nicotine is poisonous because it imitates choline, and in large doses completely borks that system. This is not entirely dissimilar to the way SSRIs (commonly used as anti-depressants) can in large doses cause serotonin syndrome, and the same mechanism that causes that makes SSRIs incredibly useful for treating depression.* Coincidentally, nicotine maybe also treats depression.
Nicotine promotes the growth of new blood vessels. If you have just had an injury this could be good thing (although I found no data on nicotine specifically, and tobacco is definitely bad for recovery), but it also makes it easier for tumors to feed themselves, which is extremely bad. It may also lead to hardening of existing blood vessels, which is pretty much always bad.
This may or may not be related to nicotine’s ability to temporarily raise blood pressure. I would super like to tell you if or how they are related or at least if they are happening on the same time scale, but I could not find a primary source that measured the immediate cardiac of nicotine (as opposed to tobacco) with a sample size of more than 16. There are lots of secondary sources that say the increase in blood pressure/narrowing of capillaries is immediate, but they seem to be scare monger sites against nicotine use for harm reduction and don’t draw a strong distinction between tobacco and nicotine. On its own this is worrisome only if it pushes your blood pressure to problematic levels, but if the process also leads to permanent hardening of blood vessels, it’s pretty concerning.
Nicotine also has some cognition boosting effects (even in people with Alzheimers)** similar to caffeine, appears to either partially treat schizophrenia or reduce the really awful side effects of anti-psychotic, and maybe Parkinson’s as well.
Based on this, I’m pretty confident that we should be investigating nicotine-derived treatments for a number of issues, and if I get Alzheimers or Parkinsons I will probably pick up a patch the next day. But I still have serious concerns about its addictiveness that would keep me from using it as an everyday cognition enhancer. But I abstain from caffeine almost 100% of the time, so clearly I’m on the low side of risk tolerance in this area.
*Probably. No one actually knows how SSRIs help.
**You remember last month when a study came out suggesting anti-cholinergics increased propensity for dementia, and I complained that they only studied drugs that affected muscarinic receptors, not nicotinic, so drugs inhibiting nicotinic receptors might be fine? This would suggest I was wrong.