Misinformation about cannabis and opioid use disorder (OUD) may increase morbidity and mortality if it leads individuals with OUD to forego evidence-based treatment. It has not been systematically evaluated whether officially designating OUD as a qualifying condition for medical cannabis is associated with cannabis dispensaries suggesting cannabis as a treatment for OUD. To examine whether state-level policies designating OUD a qualifying condition for medical cannabis are associated with more dispensaries claiming cannabis can treat OUD. This cross-sectional, mixed-methods study of 208 medical dispensary brands was conducted in 2019 using the brands' online content. The study included dispensaries operating in New Jersey, New York, and Pennsylvania, where OUD is a qualifying condition for medical cannabis, and in Connecticut, Delaware, Maryland, Ohio, and West Virginia, where this policy does not exist. Presence of OUD on the list of qualifying conditions for a state's medical cannabis program. Binary indicators of whether online content from the brand said cannabis can treat OUD, can replace US Food and Drug Administration-approved medications for OUD, can be an adjunctive therapy to Food and Drug Administration-approved medications for OUD, or can be used as a substitute for opioids to treat other conditions (eg, chronic pain). After excluding duplicates, listings for nonexistent dispensaries, and those without online content, 167 brands across 7 states were included in the analysis (44 [26.3%] in states where OUD was a qualifying condition and 123 [73.7%] in adjacent states). A dispensary listed in a directory for West Virginia was not operational; therefore, comparison states were Connecticut, Delaware, Maryland, and Ohio. In policy-exposed states, 39% (95% CI, 23%-55%) more dispensaries claimed cannabis could treat OUD compared with unexposed states (P < .001). For replacing medications for OUD and being an adjunctive therapy, the differences were 14% (95% CI, 2%-26%; P = .002) and 28% (95% CI, 14%-42%; P < .001), respectively. The suggestion that cannabis could substitute for opioids (eg, to treat chronic pain) was made by 25% (95% CI, 9%-41%) more brands in policy-exposed states than adjacent states (P = .002). In this study, state-level policies designating OUD as a qualifying condition for medical cannabis were associated with more dispensaries claiming cannabis can treat OUD. In the current policy environment, in which medical claims by cannabis dispensaries are largely unregulated, these advertisements could harm patients. Future research linking these policies to patient outcomes is warranted.