"Though nicotine doesn’t cause the rush of heroin or crack, it's biologically similar in a crucial way: it mimics a common neurotransmitter—so well that scientists named one of the acetylcholine receptors after it. Smoking regularly reduces the number and sensitivity of these “nicotinic” receptors, and requires that the user keep ingesting nicotine just to maintain normal brain function. There are a shocking 50,000,000 nicotine addicts in the US, and one in every five deaths nationwide are the result of smoking."
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