[C]alling various drugs "poisons" as if this counts for something is foolish. By this standard basically everything, including water, is a poison, it just depends on the dose. Rather we could say that nicotine is a drug with an unusually low margin of error, meaning a fatal dose is only a small multiple of an active dose. As a counter-example, we could consider LSD, which has an unusually high margin of error, despite its reputation. According to this study there have been no well-documented fatal overdoses, ever, and some people have taken unbelievable quantities (on the order of 1000 times a recreational dose) accidentally and made a full recovery. Try that with nicotine or alcohol and you would be dead as a hammer. Anyway, that’s not to particularly recommend LSD, it’s just to say that trying to label a drug as either poisonous or not doesn’t actually tell us much worth knowing.
A reader does make a recommendation:
Yes, nicotine's poison in larger doses. But like a lot of traditionally-used plant alkaloids, it's also therapeutic. I've been using snus, a Swedish form of smokeless tobacco, which is fairly safe (honest! – look it up), and have found it very helpful for mood and for concentration. I think I'm self-medicating my cyclothymia. It's mostly harmless and has been a real quality of life improvement. I'm glad plants make such useful things.