If you ask people why they smoke you often get some interesting answers. They may say that it helps them to relax or concentrate on a difficult problem. Perhaps it gives them something to do with their hands. Some say they enjoy smoking whilst others acknowledge that is a habit that is hard to break. 

The truth is much simpler- smokers are addicted to the drug nicotine. Furthermore, cigarettes are designed as efficient nicotine delivery systems. Smokers are very good at maintaining their desired blood nicotine levels by regulating cigarette consumption. 

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So what is so special about nicotine and why is it so addictive? Nicotine is an insecticide, admittedly a natural one, and is a very potent poison indeed. Nicotine has evolved as the tobacco plants way of avoiding being eaten by insects. 

Unfortunately for humans, the structure of nicotine is very similar to the neurotransmitter, acetylcholine. Acetylcholine is involved in the pleasure and reward pathways in the brain. Within 10 seconds of puffing on a cigarette, nicotine hits and influences the brain producing a subtle high. Interestingly, the influence nicotine has on the brain depends on the dose. 

At low doses it acts as a stimulant and increases concentration, at higher doses, it acts as a relaxant. The pharmacological effects of nicotine on the brain are complex and influence several pleasure pathways, in a way similar to other addictive drugs like amphetamines. Nicotine is not the only addictive compound in cigarette smoke. Research has shown that acetaldehyde, also present in tobacco, acts in concert with nicotine to increase nicotine's pleasurable properties.