Psychoactive substance use disorders are best understood by using the infectious disease model of interactions among pathogenic agents (psychoactive substances), hosts (users), and environmental factors. The complementary roles of an agents abuse liability and its withdrawal potential have been previously described. Many individuals can rapidly become dependent on cocaine or stimulants even in the absence of observable physical withdrawal, while others can become dependent on benzodiazepines despite their relatively low abuse liability.
Although accurate phenotyping of an "addictive personality" has proved elusive, many dispositional, genetic, and environmental factors render some users more vulnerable to substance use disorders. If the primary care physician can identify those patients at risk for the development of dependence, he or she can perhaps prevent users from becoming regular users, regular users from becoming harmful ones, and harmful users from becoming dependent.
For unknown reasons, the state induced by psychoactive substances is more intrinsically rewarding to some persons than to others. The only way to find out is to take a careful history: to ask about use of each class of drug and its effects. For example, stimulants cause some patients to feel energized or euphoric but others to feel unpleasantly excited or jittery.
For a variety of cultural, social, and genetic reasons, men are more likely than women to develop substance use disorders. For example, consuming alcohol is commonly held to be more compatible with masculinity than femininity, drinking to intoxication has become an adolescent male "rite of passage" in colleges and other settings, and sex differences in the amount and activity of alcohol dehydrogenase enable males to tolerate higher doses of alcohol without intoxication. Such factors increase the likelihood that males will get more repeated exposures to alcohol than will females.
A positive family history of any substance use disorder suggests strong genetic or environmental loading for the development of dependence. Patients with a personal history of drug use and a current psychiatric disorder are at increased risk; the more severe the psychopathology, the higher the risk. Environmental factors also play a role in the development of dependence, over and above such obvious influences as the availability, accessibility, and affordability of psychoactive substances and the social acceptance or stigmatization of their use. For example, jobs in the hotel and restaurant trades, public relations, advertising, and entertainment, as well as unskilled, seasonal, and day labor, are associated with increased risk.
In general, the greater the psychosocial burden of one who has become a regular user of an agent, the greater the likelihood of developing dependency. For example, persons who drink regularly are much more likely to become alcohol-dependent on becoming widowed, divorced, retired, or disabled.
Finally, a change in the preparation or route of administration of an agent can be highly predictive of dependence. Persons who convert from using cocaine or heroin intranasally to smoking it are likely to become dependent in weeks to months, and those who convert from smoking to intravenous use are likely to become dependent in days to weeks.
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