"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."
Source
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.
