Just under a quarter of all adults in the United States were current cigarette smokers as of 1999. Although it is illegal in every state to sell cigarettes to people under age 18, about 28 percent of high-school students and 9 percent of middle-school students were smokers as well. In addition, about 6 percent of adult males and 12 percent of high-school boys used smokeless tobacco. No matter what their age, most tobacco users say they want to quit. The reason they have such a tough time doing so is because of nicotine, a naturally occurring, colorless liquid that is the addictive substance in tobacco. Nicotine can be as addictive as heroin or cocaine. Because it has such powerful effects, many tobacco users have to try to quit several times before they succeed.

Nicotine Addiction

Nicotine's effects on the brain are similar to those of other drugs of abuse. Nicotine activates the...