Systematic Review and Meta-analysis of Cannabis Treatment for Chronic Pain

Setting. Cannabis preparations have been used as a remedy for thousands of years in traditional medicine. Clinical use of cannabinoid substances is restricted, due to legal and ethical reasons, as well as limited evidence showing benefits. Objective. To assess the efficacy and harms of cannabis preparations in the treatment of chronic pain. Design. Systematic review and meta-analysis of double-blind randomized controlled trials that compared any cannabis preparation to placebo among subjects with chronic pain. An electronic search was made in Medline/Pubmed, Embase, and The Cochrane Controlled Trials Register (TRIALS CENTRAL) of all literature published until February 2008, as well as specific web pages devoted to cannabis. Studies were cross-checked, selected, and assessed. Results. Eighteen trials were included. The efficacy analysis (visual analog scales) displayed a difference in standardized means in favor of the cannabis arm of -0.61 (-0.84 to -0.37), with statistical homogeneity (I-2 = 0.0%; P = 0.50). For the analysis of harms, the following Odds Ratios (OR) and number needed to harm (NNH) were obtained: for events linked to alterations to perception, OR: 4.51 (3.05-6.66), NNH: 7 (6-9); for events affecting motor function, 3.93 (2.83-5.47), NNH: 5 (4-6); for events that altered cognitive function, 4.46 (2.37-8.37), NNH: 8 (6-12). Conclusions. Currently available evidence suggests that cannabis treatment is moderately efficacious for treatment of chronic pain, but beneficial effects may be partially (or completely) offset by potentially serious harms. More evidence from larger, well-designed trials is needed to clarify the true balance of benefits to harms.