Cannabis use among the younger population in Japan has been steadily increasing. The aim of the present review is to highlight recent knowledge regarding the molecular mechanisms of action and health risks associated with cannabis and synthetic cannabinoid consumption. We investigated the effects of Δ9-tetrahydrocannabinol (THC) and synthetic cannabinoids on place conditioning in ICR mice. Both Δ9-THC and synthetic cannabinoids produce a significant conditioned place preference. These rewarding effects were completely suppressed by the cannabinoid CB1 receptor type antagonist AM251. The cytotoxicological effects of Δ9-THC and synthetic cannabinoids were also characterized in the limbic forebrain of mice in primary culture in vitro. Δ9-THC and synthetic cannabinoids caused cell death in a dose-dependent manner. The rank order of cytotoxicological potency was synthetic cannabinoids>Δ9-THC and related to the agonistic activities of the CB1 receptor. A recent review on the harmful effects of cannabis use in humans reported that behavioral impairments, especially in terms of attention, memory, and complex information-processing ability, can last for many weeks after cessation of cannabis use among heavy users. In addition, cannabis use could be a risk factor for drug dependence and later psychosis among adolescents. The results of animal and human studies suggest that CB1 receptors play an important role in the expression of harmful effects of cannabis and synthetic cannabinoid use. Moreover, concern regarding increasing concentrations of Δ9-THC in cannabis in many countries has been noted, because more potent cannabis may be associated with worse adverse effects.