The plant Cannabis sativa, commonly called cannabis or marijuana, has been used for its psychotropic and mind-altering side effects for millennia. There has been growing attention in recent years on its potential therapeutic efficacy as municipalities and legislative bodies in the United States, Canada, and other countries grapple with enacting policy to facilitate the use of cannabis or its constituents for medical purposes. There are over 550 chemical compounds and over 100 phytocannabinoids isolated from cannabis, including Δ⁹-tetrahydrocannabinol (THC) and Cannabidiol (CBD). THC is thought to produce the main psychoactive effects of cannabis, while CBD does not appear to have similar effects. Studies conflict as to whether CBD attenuates or exacerbates the behavioral and cognitive effects of THC. This includes effects of CBD on THC induced anxiety, psychosis and cognitive deficits. In this article, we review the available evidence on the pharmacology and behavioral interactions of THC and CBD from pre-clinical and human studies particularly with reference to anxiety and psychosis like symptoms. Both THC and CBD, as well as other cannabinoid molecules, are currently being evaluated for medicinal purposes, separately and in combination. Future cannabis-related policy decisions should include consideration of scientific findings including the individual and interactive effects of CBD and THC.Neuropsychopharmacology accepted article preview online, 06 September 2017. doi:10.1038/npp.2017.209.