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"If there is a relationship between cannabis and other illicit drug use, we have to explain it. The two main explanations that feature in the public debate are: (1) that cannabis users are more likely to use other illicit drugs because of the pharmacological and other effects that cannabis has; and (2) that cannabis users are more likely to use other illicit drugs because the same black market supplies cannabis and other illicit drugs, so cannabis users are more likely to have access to other illicit drugs.

There is abundant evidence from surveys of adolescent drug use in the United States and elsewhere that cannabis use and the use of cocaine and heroin are associated (7). From the late 1970s to the 1990s in the United States, there was a strong relationship between regular cannabis use and the later use of heroin and cocaine. Kandel (8), for example, found that only 7% of American adolescents who had not used cannabis reported using another illicit drug. By contrast, 33% of those who reported using cannabis had used another illicit drug. Most (84%) daily cannabis users had done so and they had also used many more types of illicit drugs than their peers who had not used cannabis or who were not daily users of cannabis (8).

Almost all adolescents who have tried cocaine and heroin, had used alcohol, tobacco and cannabis in that order (10). Those who began to use alcohol and tobacco at an early age, and those became regular smokers and drinkers, were the ones who were most likely to use cannabis. In turn, it was cannabis users who began use at an early age who were the most likely to become regular cannabis users and the most likely to use hallucinogens, amphetamines and tranquillizers. The heaviest users of these drugs were, in turn, more likely to use cocaine and heroin."- The Health and Psychological effects of Cannabis Use