I don’t even know where to begin writing about nicotine.
If there is any substance that has a more “Jekyll and Hyde” profile than nicotine, I’m not aware of it. It can be a stimulant or a calmative. It can be a deadly poison, or it can be therapeutic. It can aid in spurts of energy and lead to periods of lethargy. Finally, most people know little about the substance, including their own intake, but I digress (as I am frequently wont to do).
Let’s start with the basics- nicotine is an alkaloid, and in highly concentrated doses is a strong poison. The amount taken in during smoking is nowhere near what would be necessary for toxicity, and under all but the most unusual circumstances (such as wearing a number of transdermal patches and chain-smoking cigarettes simultaneously) will rarely cause severe illness or death. It also is a vasoconstrictor, so it can elevate blood pressure and can affect many bodily systems.