A systematic search of the literature was carried out in MEDLINE, the Cochrane central register of controlled trials (CENTRAL) and clinicaltrials.gov up until November 2015. We included double-blind randomized placebo-controlled studies (RCT) of at least 2 weeks duration and with at least 9 patients per treatment arm comparing
medicinal cannabis, plant-based or synthetic
cannabinoids with placebo or any other active
drug treatment in patients with chronic
neuropathic pain. Clinical endpoints of the analyses were efficacy (more than 30 % or 50 % reduction of
pain, average
pain intensity, global improvement and health-related quality of life), tolerability (drop-out rate due to side effects, central nervous system and psychiatric side effects) and safety (severe side effects). Using a random effects model absolute risk differences (RD) were calculated for categorical data and standardized mean differences (SMD) for continuous variables. The methodological quality of RCTs was rated by the Cochrane risk of bias tool.
RESULTS: We included 15 RCTs with 1619 participants. Study duration ranged between 2 and 15 weeks. Of the studies 10 used a plant-derived oromucosal spray with
tetrahydrocannabinol/
cannabidiol, 3 studies used a synthetic
cannabinoid (2 with
nabilone and 1 with
dronabinol) and 2 studies used
medicinal cannabis. The 13 studies with parallel or cross-over design yielded the following results with 95 % confidence intervals (CI):
cannabinoids were superior to placebo in the reduction of mean
pain intensity with SMD - 0.10 (95 % CI - 0.20- - 0.00, p = 0.05, 13 studies with 1565 participants), in the frequency of at least a 30 % reduction in
pain with an RD of 0.10 [95 % CI 0.03-0.16, p = 0.004, 9 studies with 1346 participants, number needed to treat for additional benefit (NNTB) 14, 95 % CI 8-45] and in the frequency of a large or very large global improvement with an RD of 0.09 (95 % CI 0.01-0.17, p = 0.009, 7 studies with 1092 participants). There were no statistically significant differences between
cannabinoids and placebo in the frequency of at least a 50 % reduction in
pain, in improvement of health-related quality of life and in the frequency of serious adverse events. Patients treated with
cannabinoids dropped out more frequently due to adverse events with an RD of 0.04 [95 % CI 0.01-0.07, p = 0.009, 11 studies with 1572 participants, number needed to treat for additional harm (NNTH) 19, 95 % CI 13-37], reported central nervous system side effects more frequently with an RD of 0.38 (95 % CI 0.18-0.58, p = 0.0003, 9 studies with 1304 participants, NNTH 3, 95 % CI 2-4) and psychiatric side effects with an RD of 0.11 (95 % CI 0.06-0.16, p < 0.0001, 9 studies with 1304 participants, NNTH 8, 95 % CI 7-12).
CONCLUSION:
Cannabinoids were marginally superior to placebo in terms of efficacy and inferior in terms of tolerability.
Cannabinoids and placebo did not differ in terms of safety during the study period. Short-term and intermediate-term
therapy with
cannabinoids can be considered in selected patients with chronic
neuropathic pain after failure of first-line and second-line
therapies.