It is generally agreed that the more quickly a drug is delivered to the brain, the more likely it will lead to dependence. Consequently, the routes of injecting and smoking which deliver drugs relatively quickly are considered more addictive than the routes of snorting (insufflation) or swallowing, which deliver the drugs more slowly. This may explain why tobacco use (which typically involves smoking) and heroin use (which is most often injected) are the two drugs most often associated with dependence among users (Table 1). 
Although the rapid delivery of drugs to the brain may promote dependence, the actual mechanism underlying this relationship is still under debate. One explanation suggests that rapidly administered drugs promote the transition to dependence because they are more euphorigenic. In fact, some studies indicate that self-reports of a “high,” pleasantness, and drug-liking are greater when cocaine or heroin are administered intravenously rather than by insufflation, and ratings of euphoria are greater when intravenous infusions of cocaine or morphine are delivered rapidly. A difficulty with this explanation, however, is that dependence can develop in the absence of strong feelings of addiction. For example, cigarettes are not particularly euphorigenic, yet they are powerfully addictive.
Another explanation suggests that rapidly administered drugs are more addictive because they are more reinforcing. This is based on the idea that behaviors/actions that are rewarded more quickly tend to be more reinforcing. Research on drug administration and reinforcement has been based largely on a series of studies that show that increasing the speed of intravenous administration enhances the ability of cocaine and nicotine to support drug self-administration behavior in monkeys. Researchers have questioned whether this work is sufficient to draw confident conclusions.
A third explanation is that dependence is due, at least in part, to the ability of drugs of abuse to reorganize brain regions involved in reward and motivation, such as the dorsal and ventral striatum, and brain regions involved in the inhibitory control of behavior, such as the prefrontal cortex. This possibility is under study.