"While the findings of this study indicate that early cannabis use is associated with increased risks of progression to other illicit drug use and drug abuse/dependence, it is not possible to draw strong causal conclusions solely on the basis of the associations shown in this study."
Lynskey, Michael T., PhD, et al. Escalation of Drug Use in Early-Onset Cannabis Users vs Co-twin Controls. Journal of the American Medical Association, Vol. 289 No. 4, January 22/29, 2003.