@Eddie51
//Nicotine is more addictive than Heroin is what I meant to say, sorry.//
If I remember rightly, Nicotine triggers the release of endorphins by the brain, producing a 'high'. At least it does the first half a dozen times. Cruelly, for the rest of their lives, smokers have to indulge to lift themselves from a 'low' back to a 'normal' level. Even an evening chain-smoking in the pub fails to produce the kind of 'high' they once had.
Heroin mimics the natural endorphins but, worse still, replaces them. The brain loses the ability to make the natural compound and the user begins to require the drug just to maintain normal brain functioning. Their lows are devastatingly low, using just raises them to 'functional' and, like with alcohol, highs require steadily higher doseages (the liver gets ever more effective at breaking intoxicants down, until it fails), which inevitably leads to disaster.
If only we regarded tobacco pushers the same way we regard drug pushers, eh?