Abstract Background Debate about medical cannabis legalization are typically informed by three beliefs: (1) cannabis has medical effects, (2) medical cannabis is addictive and (3) medical cannabis legalization leads to increased used of cannabis for recreational purposes (spillover effects). We examined how strongly these beliefs are associated with public support for medical cannabis legalization and whether this association differs across divergent medical cannabis policy regimes. Methods Robust regression analysis was used to analyse data derived from two nationally representative samples of adults participating in comparable cross-sectional online surveys in one country where medical cannabis smoking is illegal (Norway, n  = 2175, 51 % male) and in one country where medical cannabis smoking is legal (Israel, n  = 648, 49 % male). Results The belief that cannabis has medical benefits was more strongly related to support for medical cannabis legalization than were beliefs about addiction and spillover effects. While the support for medical cannabis legalization was stronger in Israel than in Norway (78 vs. 51 %, p