NNicotine is a widely used stimulant that has psychoactive properties. Once nicotine enters the bloodstream, it is transported to the brain, where it binds to receptors that increase the levels of several neurotransmitters— such as dopamine — which activate the reward system. These actions are largely responsible for the strongly reinforcing and highly addictive effects of nicotine.
There are approximately one billion people in the world who smoke tobacco of which nicotine is a constituent. It is estimated that half of smokers (and one-third of former smokers) are dependent on nicotine. Recent US data suggests that the rates of daily smoking and the number of cigarettes smoked per day are declining. Some people groups are more likely to smoke than the average population, such as those with low education or low socioeconomic status and those with mental illness. Rates in these groups are stable or increasing. Men smoke at higher rates than do women and score higher on dependence criteria.