Systematic review on the effectiveness of medical cannabis for the treatment of chronic pain

Abstract

Background Chronic pain is a highly prevalent condition worldwide, impacting both the quality of life of patients and the healthcare system. Despite the conventional treatments available, many patients still experience inadequate pain relief due to the complexity of the condition. This has led to a growing interest in alternative treatments, such as medical cannabis, however, research concerning its effectiveness, that is how well it works in the real-world, remains inconclusive and variable. Aim The main aim of this review is to evaluate the effectiveness of medical cannabis for the treatment of chronic pain. Where possible the effectiveness of the various formulations and types of medical cannabis is also investigated. Methodology Following an initial scoping literature review, a protocol for the systematic review was drawn up and published on PROSPERO. A comprehensive search was conducted from September 2023 to January 2024 across numerous databases including MEDLINE – ProQuest, Cumulative Index to Nursing and Allied Health Literature (CINAHL), Cochrane Library, Web of Science, Scopus, PsycINFO, PubMed, and Google Scholar (for grey literature). The search utilised terms related to medical cannabis, chronic pain, and effectiveness. The papers retrieved were selected and reviewed based on predefined eligibility criteria. Results Twenty studies met the inclusion criteria: three systematic reviews with meta-analyses, one randomised controlled trial (RCT), and 16 observational studies. These studies were conducted in various countries, with treatment durations ranging from three months to a year, except for the RCT which lasted eight 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. Nabiximols (NBX) and dronabinol (DRO) were the most commonly used formulations of medical cannabis, administered as oromucosal spray and oral capsules, respectively. Other forms included sublingual oils, oral capsules, gums, lozenges, inflorescence, decoction and inhaled preparations. Most studies reported statistically significant pain reduction with medical cannabis use. Six studies focused on neuropathic pain, with all but one (the double-blind RCT) showing improvement. The RCT found that oral capsules containing CBD, THC, and a combination of both did not reduce pain more than placebo. Two observational studies compared medical cannabis to other treatments, finding NBX more effective than long-acting opioids (LAO) and DRO for neuropathic pain. In fibromyalgia, one study reported statistically significant pain improvement over 12 months, while another found statically non-significant improvements at 12 months follow-up. Other studies and systematic reviews generally reported statistically significant pain reduction with medical cannabis, with one systematic review observing improvement with prolonged use. The quality of the included studies was evaluated using the Critical Appraisal Skills Programme (CASP) Checklists, according to the study designs. Most observational studies were of medium to high quality, although some had methodological limitations. The RCT and the systematic reviews were also of high quality but one systematic review lacked a clear research question and robust search strategy, limiting its reliability. Conclusion This systematic review found inconclusive evidence regarding the effectiveness of medical cannabis for chronic pain. [...]