Nerves, metabolic rate stimulation

Repeated exposure, as occurs in habitual smokers, leads to an increased rate of metabolism of nicotine and decreased sensitivity of the receptors to the nicotine. More nicotine is therefore required to satisfy the needed stimulation, and tolerance develops. This is a separate effect from addiction which seems to be due to increased levels of a substance called dopamine in the brain. This is due to the nicotine activating nerve cells which results in increased release of dopamine and causes a reduction in the amount of an enzyme that destroys dopamine. It seems that other addictive drugs may also work by increasing dopamine levels. 

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