"The causal significance of this sequence of initiation into drug use remains controversial. The hypothesis that it represents a direct effect of cannabis use upon the use of the later drugs in the sequence is the least compelling. There is better support for two other hypotheses which are not mutually exclusive: that there is a selective recruitment into cannabis use of nonconforming adolescents who have a propensity to use other illicit drugs; and that once recruited to cannabis use, the social interaction with other drug using peers, and exposure to other drugs when purchasing cannabis on the black-market, increases the opportunity to use other illicit drugs"
Hall, W., Room, R. & Bondy, S. WHO Project on Health Implications of Cannabis Use: A Comparative Appraisal of the Health and Psychological Consequences of Alcohol, Cannabis, Nicotine and Opiate Use. Geneva, Switzerland: World Health Organization, August 28, 1995.