 Background: Cannabis sativa contains over sixty cannabinoids, including delta-9-tetrahydrocannabinol THC and cannabidiol CBD. CBD lacks psychoactivity and may attenuate the subjective effects produced by THC. The ratio of THC:CBD in illicit cannabis in the US is approximately 20:1. SativexRegistered Trademark, a whole-plant cannabinoid extract oromucosal spray, contains THC and CBD in a ratio of nearly 1:1. It was approved by Health Canada in 2005 as a prescription medication for neuropathic pain in multiple sclerosis MS and will be evaluated in Phase III trials in the U.S. in patients with advanced cancer for the treatment of pain refractory to opiates. No studies have examined changes in regional brain activity with functional magnetic resonance imaging fMRI during completion of cognitive tasks or affective measures after administration of SativexRegistered Trademark. 