Medical cannabis for chronic pain : a systematic review of effectiveness

Abstract

Background: Chronic pain significantly impacts patients’ quality of life and places a substantial burden on healthcare systems worldwide. Despite the availability of conventional treatments, many patients continue to experience inadequate pain relief, which has led to increased interest in alternative therapies such as medical cannabis. Aims: The primary aim of this systematic review is to evaluate the effectiveness of medical cannabis for the treatment of chronic pain, exploring various formulations and types of medical cannabis. Methodology: A systematic review protocol was registered on PROSPERO: CRD 42023447171. A comprehensive literature search was conducted across multiple databases, including MEDLINE, CINAHL, Cochrane Library, Web of Science, Scopus, PsycINFO, PubMed, and Google Scholar. The search utilised terms related to medical cannabis, chronic pain, and effectiveness. Studies were reviewed and selected based on predefined inclusion and exclusion criteria. Two independent reviewers were involved throughout the selection process. The methodological quality of the articles was assessed using Critical Appraisal Skills Programme (CASP) checklists. Results: A total of 20 studies met the inclusion criteria: three systematic reviews with meta-analysis, one randomised controlled trial (RCT), and 16 observational studies. These studies were conducted in various countries, with treatment durations ranging from 3 months to a year, except for the randomised controlled trial, which lasted 8 weeks. Participants were mostly older adults with various chronic pain types, including chronic cancer pain and chronic non-cancer pain such as neuropathic pain, fibromyalgia, and arthritis. The review analysed different formulations of medical cannabis. Nabiximols (NBX) and dronabinol (DRO) were the most commonly used, administered as oromucosal sprays and oral capsules, respectively. Other forms included sublingual oils, oral capsules, gums, lozenges, and inhaled preparations. The effectiveness of medical cannabis varied depending on the type of pain and the formulation used, with balanced tetrahydrocannabinol/cannabidiol (THC/CBD) products showing promise for chronic cancer pain and as an adjunct for neuropathic pain. Conclusion: This systematic review found inconclusive evidence regarding the effectiveness of medical cannabis for chronic pain. Drawing definitive conclusions was challenging due to variability in the types of chronic pain and cannabis formulations analysed in the selected studies, which limited generalisability. Furthermore, promising evidence was found suggesting that balanced THC/CBD products may be effective for chronic cancer-related pain and as an add-on for neuropathic pain, particularly in the formulation of an oromucosal spray. However, further research is required to draw firm and definitive conclusions.