brain damage/changes from nicotine?
There's an oft-parroted claim that nicotine changes the juvenile brain, or causes irreperable damage in its development. This of course is bunk, based on misrepresenting rodent studies.
The human brain in 14-19 year olds is nothing like that of rats or mice in their
equivalent "adolescent" stage (4-12 weeks old).
And if nicotine had such lasting effects on the human brain, "would we not have seen it?" (Tobacco had been around for a century.)
- Myth #6: The nicotine in JUUL causes brain damage in teenagers
- Q4.6: Does nicotine damage the developing adolescent brain?
- Vaping: what people are getting wrong | The Economist
nicotine == poison
The origin assumption is certainly that nicotine is a plant neurotoxin for insects. Which of course has much less implications for humans, where it acts more like a nutrient/hormone. (When oxidized as nicotinic acid; it's also called niacin, formerly known as a vitamin b3 form [term not used anymore]).
nicotine == heroin
The dated hyperbole on the heroin comparison notwithstanding, that's not to say that nicotine couldn't induce an addiction by itself (even isolated from tobacco). But the implication that it changes the human brain, even for individuals where it potentially promotes other substance use, is still a bit farfetched. 
media scare phrases
- "nicotine = brain poison🟥" (TFCA/flavorshookkids, PR agency)
- sci-fi brain worms🟥 (CTFK)
- "harm adolescent brain development🟥" (CDC-OSH)
- "Studies in rats🟧 have shown that chronic nicotine exposure during adolescence can diminish cognitive functions" (TI)