According to a recent study by Yu-Wei Luke Chu in the Journal of Law & Economics, the answer is not just “No,” but that medical marijuana laws may actually decrease heroin use as consumers substitute the legal marijuana for heroin. Below is the abstract:
Medical marijuana laws generate significant debate regarding drug policy. For instance, if marijuana is a complement to hard drugs, then these laws would increase the usage not only of marijuana but also of hard drugs. In this paper I study empirically the effects of medical marijuana laws by analyzing data on drug arrests and treatment admissions. I find that medical marijuana laws increase these proxies for marijuana consumption by around 10–15 percent. However, there is no evidence that cocaine and heroin usage increases. From the arrest data, the estimates indicate a 0–15 percent decrease in possession arrests for cocaine and heroin combined. From the treatment data, the estimates show a 20 percent decrease in admissions for heroin-related treatment, although there is no significant effect for cocaine-related treatment. These results suggest that marijuana may be a substitute for heroin, but it is not strongly correlated with cocaine.