Tobacco—What it Is and What it Does - The Addictive Nature Of Nicotine
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According to a 2004 Gallup Poll, 78% of smokers believed they were addicted to cigarettes. Is tobacco addictive? In The Health Consequences of Smoking—Nicotine Addiction: A Report of the Surgeon General (Rockville, MD: U.S. Department of Health and Human Services, 1988), researchers examined this question. They determined that the pharmacological (chemical/physical) effects and behavioral processes that contribute to tobacco addiction are very similar to those that contribute in the addiction to drugs such as heroin and cocaine. Nicotine is considered by many researchers to be as potentially addictive as cocaine and heroin, and it can create dependence quickly in some users.
Researchers have also discovered that some cigarettes have a "kick," containing thirty-five times more freebase nicotine than other cigarettes. According to a July 2003 report in the Journal of Chemical Research in Toxicology, the danger of this freebase nicotine is that it is in a volatile, uncombined form. This form is absorbed by the lungs and brain at a faster rate than standard forms of nicotine. Researchers have even referred to this raw form of nicotine as "crack nicotine," because it potentially has the same addictive quality as crack cocaine. (A drug's addictiveness is measured by the speed at which it reaches the brain.)
Cigarette smoking results in rapid distribution of nicotine throughout the body, reaching the brain within ten seconds of inhalation. But the intense effects of nicotine disappear in a few minutes, causing smokers to continue smoking frequently throughout the day in order to maintain its pleasurable effects and to prevent withdrawal. Tolerance develops after repeated exposure to nicotine, and higher doses are required to produce the same initial stimulation. Because nicotine is metabolized fairly quickly, disappearing from the body in a few hours, some tolerance is lost overnight. Smokers often report that the first cigarette of the day is the most satisfying. The more cigarettes smoked during the day, the more tolerance develops, and the less effect subsequent cigarettes have.
Is There a Genetic Basis for Addiction?
Recently, scientists identified a gene that appears to influence whether some people are more likely to become addicted to nicotine than others. Rachel F. Tyndale and her colleagues at the University of Toronto in Ontario, Canada, compared 244 habitual smokers with 184 people who had tried tobacco but had not become addicted. They found that those in the nonaddicted group were much more likely to have inactive versions of this gene. Among those who smoked regularly, those with inactive versions of the gene smoked fewer cigarettes.
Previous studies have identified one or two other genes thought to play a role in nicotine addiction. Some genetics experts believe that the basis of nicotine addiction is more complex than simply one or two genes. Others believe that genetic factors account for only about half of the susceptibility to nicotine addiction; the other half depends on the environment in which someone is raised.
Results from the Collaborative Study on the Genetics of Alcoholism (COGA) support the hypothesis that some common genetic factors are involved in the susceptibility for developing both alcohol and nicotine addiction ("Co-occurring Risk Factors for Alcohol Dependence and Habitual Smoking," Alcohol Research and Health, vol. 24, no. 4, 2000). Moreover, twin studies have supported the role of common genetic factors in the development of both disorders.
Nicotine May Not Be the Only Addictive Substance
Research results suggest that nicotine may not be the only psychoactive (mind-altering) ingredient in tobacco. Some as-yet-unknown compound in cigarette smoke decreases the levels of monoamineoxidase (MAO), an enzyme responsible for breaking down the brain chemical dopamine. The decrease in MAO results in higher dopamine levels and may be another reason that smokers continue to smoke—to sustain the high dopamine levels that result in pleasurable effects and the desire for repeated cigarette use.
One issue that complicates any efforts by a long-time smoker to quit is nicotine withdrawal, which is often referred to as craving. This urge for nicotine is not well understood by researchers. Withdrawal may begin within a few hours after the last cigarette. High levels of craving may persist six months or longer, according to the National Institute on Drug Abuse. In addition to craving, withdrawal can include irritability, attention deficits, interruption of thought processes, sleep disturbances, and increased appetite.
Some researchers have pointed out the behavioral aspects involved in smoking as well. The purchasing, handling, and lighting of cigarettes may be just as pleasing psychologically to the user as the physical properties of tobacco itself.
As with other drugs, however, not all users of tobacco become dependent. When David Mendez, Assistant Professor of Public Health at the University of Michigan, was analyzing smoking statistics from surveys conducted for the CDC, he discovered that 18% of the country's smokers said they smoked, but not on a daily basis. Some researchers even believe that occasional smokers constitute a growing trend.