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Definitions of an addictive substance may vary slightly, but 

 they all embody some key criteria: first, compulsive use, often 

 despite knowing the substance is harmful; second, a psychoactive 

 effect -- that is, a direct chemical effect in the brain; third, 

 what researchers call reinforcing behavior that conditions 

 continued use^''. (Chart A) In addition, withdrawal symptoms 

 occur with many drugs and occur in many cigarette smokers who try 

 to quit. These are hallmarks of an addictive substance and 

 nicotine meets them all. 



When a smoker inhales, once absorbed in the bloodstream, 

 nicotine is carried to the brain in only 7-9 seconds^', setting 

 off a biological chain reaction that is critical in establishing 

 and reinforcing addiction. 



Over the past few years, scientists have generated a 

 tremendous amount of information on the similarities among 

 different addictive substances. Some crucial information has 

 come from the fact that, in a laboratory setting, animals will 

 self -administer addictive, substances . This self -administration 

 may involve the animal pushing a lever or engaging in other 

 actions to get repeated doses of the addictive substance. With 

 very few exceptions, animals will self -administer those drugs 

 that are considered highly addictive in humans, including 

 morphine and cocaine, and will not self -administer those drugs 

 that are not considered addictive ^* , ^^ . 



Understanding that animals will self -administer addictive 

 substances has fundamentally changed the way that scientists view 



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