Objective To assess the efficacy and harms of adding medical cannabis to prescription opioids among people living with chronic pain.
Design Systematic review.
Data sources CENTRAL, EMBASE and MEDLINE.
Main outcomes and measures Opioid dose reduction, pain relief, sleep disturbance, physical and emotional functioning and adverse events.
Study selection criteria and methods We included studies that enrolled patients with chronic pain receiving prescription opioids and explored the impact of adding medical cannabis. We used Grading of Recommendations Assessment, Development and Evaluation to assess the certainty of evidence for each outcome.
Results Eligible studies included five randomised trials (all enrolling chronic cancer-pain patients) and 12 observational studies. All randomised trials instructed participants to maintain their opioid dose, which resulted in a very low certainty evidence that adding cannabis has little or no impact on opioid use (weighted mean difference (WMD) −3.4 milligram morphine equivalent (MME); 95% CI (CI) −12.7 to 5.8). Randomised trials provided high certainty evidence that cannabis addition had little or no effect on pain relief (WMD −0.18 cm; 95% CI −0.38 to 0.02; on a 10 cm Visual Analogue Scale (VAS) for pain) or sleep disturbance (WMD −0.22 cm; 95% CI −0.4 to −0.06; on a 10 cm VAS for sleep disturbance; minimally important difference is 1 cm) among chronic cancer pain patients. Addition of cannabis likely increases nausea (relative risk (RR) 1.43; 95% CI 1.04 to 1.96; risk difference (RD) 4%, 95% CI 0% to 7%) and vomiting (RR 1.5; 95% CI 1.01 to 2.24; RD 3%; 95% CI 0% to 6%) (both moderate certainty) and may have no effect on constipation (RR 0.85; 95% CI 0.54 to 1.35; RD −1%; 95% CI −4% to 2%) (low certainty). Eight observational studies provided very low certainty evidence that adding cannabis reduced opioid use (WMD −22.5 MME; 95% CI −43.06 to −1.97).
Conclusion Opioid-sparing effects of medical cannabis for chronic pain remain uncertain due to very low certainty evidence.
PROSPERO registration number
- pain management
- cancer pain
- general medicine (see internal medicine)
Data availability statement
Data are available on reasonable request to the corresponding author at: email@example.com.
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Contributors JWB, AN and GHG conceived and designed the study. RJC performed the literature search. AN, AM, YS, VA and YR selected the studies, extracted the relevant information, and assessed the risk of bias of selected studies. AN synthesised the data. AN wrote the first draft of the paper. AN, JWB, GHG and TA critically revised the manuscript for important intellectual content. AN, JWB, LT, GHG, MB and DNB interpreted the findings. JWB, LT and GHG provided methodological support. All authors reviewed the paper and approved the final version. The corresponding author attests that all listed authors meet authorship criteria and that no others meeting the criteria have been omitted.
Funding The authors have not declared a specific grant for this research from any funding agency in the public, commercial or not-for-profit sectors.
Competing interests None declared.
Provenance and peer review Not commissioned; externally peer reviewed.
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