Currently, 29 states and Washington, DC, have passed laws to legalize medical marijuana. Although evidence for the effectiveness of marijuana or its extracts for most medical indications is limited and in many cases completely lacking, there are a handful of exceptions. For example, there is increasing evidence for the efficacy of marijuana in treating some forms of pain and spasticity, and 2 cannabinoid medications (dronabinol and nabilone) are approved by the US Food and Drug Administration for alleviating nausea induced by cancer chemotherapy. A systematic review and meta-analysis by Whiting et al1 found evidence, although of low quality, for the effectiveness of cannabinoid drugs in the latter indication. The antinausea effects of tetrahydrocannabinol (THC), the main psychoactive ingredient in marijuana, are mediated by the interactions of THC with type 1 cannabinoid (CB1) receptors in the dorsal vagal complex. Cannabidiol, another cannabinoid in marijuana, exerts antiemetic properties through other mechanisms. Nausea is a medically approved indication for marijuana in all states where medical use of this drug has been legalized.