In the current study we use a synthetic control group design to estimate the causal effect of a medical marijuana initiative on suicide risk. In 1996, California legalized marijuana use for medical purposes. Implementation was abrupt and uniform, presenting a “natural experiment.” Utilizing a panel dataset containing annual frequencies of Total, gun, and non-gun suicides aggregated by state for the years 1970-2004, we construct a control time series for California as a weighted combination of the 41 states that did not legalize marijuana during the analysis period. Post-intervention differences for California and its constructed control time series can be interpreted as the effects of the medical marijuana law on suicide. Significance of the effects were assessed with permutation tests. Our findings suggest that California’s 1996 legalization resulted in statistically significant (p<.05) reductions in suicides and gun suicides, but only a non-significant reduction in non-gun suicides (p≥.488). Since the effect for non-gun suicides was indistinguishable from chance, we infer that the overall causal effect was realized through gun suicides. The mechanism could not be determined, however. Participation in the medical marijuana program legally disqualifies participants from purchasing guns. But since most suicides involve guns, it is possible that the effect on total suicide is driven by gun suicide alone.