More evidence that marijuana is a gateway drug has emerged. A recent study in Addictive Behaviors found that individuals who began using marijuana prior to age 15 were 1.6–4.2 times more likely to have had an opioid use disorder in the past year and 1.9–2.3 times more likely to have had an opioid use disorder in their lifetime. The study, led by researchers at the Washington University School of Medicine, also found that these individuals were 3.0–5.8 times more likely to have injected heroin and 2.2–3.0 times more likely to have used illegally manufactured fentanyl. Given the concerning association between early marijuana use and subsequent opioid misuse, policymakers should oppose all efforts to legalize and commercialize marijuana so as to close off this dangerous pathway.