addiction - Varied forms of addiction - Physical dependency
Physical dependence on a substance is defined by the appearance of characteristic withdrawal symptoms when the substance or behavior is suddenly discontinued. While opioids, benzodiazepines, barbiturates, alcohol and nicotine are all well known for their ability to induce physical dependence, other categories of substances share this property and are not considered addictive: cortisone, beta-blockers and most antidepressants are examples. So, while physical dependency can be a major factor in the psychology of addiction and most often becomes a primary motivator in the continuation of an addiction, the initial primary attribute of an addictive substance is usually its ability to induce pleasure, although with continued use the goal is not so much to induce pleasure as it is to relieve the anxiety caused by the absence of a given addictive substance, causing it to become used compulsively. A notable exception to this is nicotine. Users report that a cigarette can be pleasurable, but there is a medical consensus that the user is likely fulfilling his/her physical addiction and, therefore, is achieving pleasurable feelings relative to his/her previous state of physical withdrawal. Further, the physical dependency of the nicotine addict on the substance itself becomes an overwhelming factor in the continuation of most users' addictions. Although 35 million smokers make an attempt to quit every year, fewer than 7% achieve even one year of abstinence (from the NIDA research report on nicotine addiction).

Some substances induce physical dependence or physiological tolerance - but not addiction - for example many laxatives, which are not psychoactive; nasal decongestants, which can cause rebound congestion if used for more than a few days in a row; and some antidepressants, most notably venlafaxine, paroxetine and sertraline, as they have quite short half-lives, so stopping them abruptly causes a more rapid change in the neurotransmitter balance in the brain than many other antidepressants. Many non-addictive prescription drugs should not be suddenly stopped, so a doctor should be consulted before abruptly discontinuing them.

The speed with which a given individual becomes addicted to various substances varies with the substance, the frequency of use, the means of ingestion, the intensity of pleasure or euphoria, and the individual's genetic and psychological susceptibility. Some alcoholics report they exhibited alcoholic tendencies from the moment of first intoxication, while most people can drink socially without ever becoming addicted. Studies have demonstrated that opioid dependent individuals have different responses to even low doses of opioids than the majority of people, although this may be due to a variety of other factors, as opioid use heavily stimulates pleasure-inducing neurotransmitters in the brain. The vast majority of medical professionals and scientists agree that if one uses strong opioids on a regular basis for even just a short period of time, one will most likely become physically dependent. Nonetheless, because of these variations, in addition to the adoption and twin studies that have been well replicated, much of the medical community is satisfied that addiction is in part genetically moderated. That is, one's genetic makeup may regulate how susceptible one is to a substance and how easily one may become psychologically attached to a pleasurable routine.

Eating disorders are complicated pathological mental illnesses and thus are not the same as addictions described in this article. Eating disorders, which some argue are not addictions at all, are driven by a multitude of factors, most of which are highly different than the factors behind addictions described in this article.

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