The United States is presently going through two substantial changes as it relates to drug use—more states are legalizing marijuana for the purposes of medical treatment and prescription opioid abuse is on the rise, resulting in heroin use rates nearly quadrupling over the past fifteen years. Historically, marijuana has been viewed as a gateway drug. Recent research suggests that medical marijuana legalization has decreased incidence of prescription and other opioid use and overdose. Examining heroin use data and other control variables, we test the effect of medical marijuana legalization on heroin use to determine whether medical marijuana is a gateway drug or substitute for heroin. We find that medical marijuana legalization has a generally negative, but statistically insignificant effect on heroin use rates. This suggests that while the legalization of medical marijuana will not lead to a reduction in heroin use, medical marijuana is not a gateway drug for heroin.