Why does the rapid delivery of drugs to the brain promote addiction?

It is widely accepted that the more rapidly drugs of abuse reach the brain the greater their potential for addiction. This might be one reason why cocaine and nicotine are more addictive when they are smoked than when they are administered by other routes. Traditionally, rapidly administered drugs are thought to be more addictive because they are more euphorigenic and/or more reinforcing. However, evidence for this is not compelling. We propose an alternative (although not mutually exclusive) explanation based on the idea that the transition to addiction involves drug-induced plasticity in mesocorticolimbic systems, changes that are manifested behaviourally as psychomotor and incentive sensitization. Recent evidence suggests that rapidly administered cocaine or nicotine preferentially engage mesocorticolimbic circuits, and more readily induce psychomotor sensitization. We conclude that rapidly delivered drugs might promote addiction by promoting forms of neurobehavioural plasticity that contribute to the compulsive pursuit of drugs.