Cannabinoids have also a profound anti inflammatory effect, mainly through the CB2 receptor. Cell mediated immunity may be impaired in chronic marijuana users. And a potent anti-inflammatory effect of cannabis was observed in rats . Studying the functional roles of the endocannabinoid system in immune modulation reveals that there are no major immune events which do not involve the endocannabinoid system. Cannabinoids shift the balance of pro-inflammatory cytokines and anti-inflammatory cytokines towards the T-helper cell type 2 profiles (Th2 phenotype), and suppress cell-mediated immunity whereas humoral immunity may be enhanced. They are therefore used for various inflammatory conditions including rheumatoid arthritis and asthma. In a mouse model of colitis cannabinoids were found to ameliorate inflammation and there are many anecdotal reports about the effect of cannabis in inflammatory bowel disease. However, there are no methodical reports of the effect of cannabis on inflammatory bowel disease. The aim of the proposed study is to examine in a double blind placebo controlled fashion the effect of smoking cannabis on disease activity in patients with IBD.