In the United States as of 1999, there were about 57 million cigarette smokers-representing 25 percent of the adult population. Another 5 percent (men) use smokeless tobacco (chewing tobacco or snuff). Most (70-80%) say they would like to quit. Unfortunately, they are dependent on (addicted to) nicotine, an alkaloid that makes it difficult to stop using tobacco. Most of them will have to try to quit several times before they are successful. Both the direct effects of nicotine on the body and behavioral associations with those effects learned over the years of tobacco use keep people going back for more even when they want to quit.

The role of nicotine in tobacco use is complex. Nicotine acts on the body directly to produce effects such as pleasure, arousal, enhanced vigilance, relief of anxiety, reduced hunger, and body-weight reduction. It may also reverse the withdrawal who is symptoms that occur in...