Marijuana is not a gateway drug
"Our results indicate a moderate relation between early teen marijuana use and young adult abuse of other illicit substances; however, this association fades from statistical significance with adjustments for stress and life-course variables. Likewise, our findings show that any causal influence of teen marijuana use on other illicit substance use is contingent upon employment status and is short-term, subsiding entirely by the age of 21. In light of these findings, we urge U.S. drug control policymakers to consider stress and life-course approaches in their pursuit of solutions to the 'drug problem.'"
Van Gundy, Karen and Rebellon, Cesar J., "Life-course Perspective on the “Gateway Hypothesis” Journal of Health and Social Behavior (Thousand Oaks, CA: American Sociological Association, September 2010), p. 244.