Objectives To evaluate the safety and effectiveness of medical cannabis (MC) in reducing pain and concurrent medications in patients with cancer.

Methods This study analysed data collected from patients with cancer who were part of the Quebec Cannabis Registry. Brief Pain Inventory (BPI), revised Edmonton Symptom Assessment System (ESAS-r) questionnaires, total medication burden (TMB) and morphine equivalent daily dose (MEDD) recorded at 3-month, 6-month, 9-month and 12-month follow-ups were compared with baseline values. Adverse events were also documented at each follow-up visit.

Results This study included 358 patients with cancer. Thirteen out of 15 adverse events reported in 11 patients were not serious; 2 serious events (pneumonia and cardiovascular event) were considered unlikely related to MC. Statistically significant decreases were observed at 3-month, 6-month and 9-month follow-up for BPI worst pain (5.5±0.7 baseline, 3.6±0.7, 3.6±0.7, 3.6±0.8; p<0.01), average pain (4.1±0.6 baseline, 2.4±0.6, 2.3±0.6, 2.7±0.7; p<0.01), overall pain severity (3.7±0.5 baseline, 2.3±0.6, 2.3±0.6, 2.4±0.6; p<0.01) and pain interference (4.3±0.6 baseline, 2.4±0.6, 2.2±0.6, 2.4±0.7, p<0.01). ESAS-r pain scores decreased significantly at 3-month, 6-month and 9-month follow-up (3.7±0.6 baseline, 2.5±0.6, 2.2±0.6, 2.0±0.7, p<0.01). THC:CBD balanced strains were associated with better pain relief as compared with THC-dominant and CBD-dominant strains. Decreases in TMB were observed at all follow-ups. Decreases in MEDD were observed at the first three follow-ups.

Conclusions Real-world data from this large, prospective, multicentre registry indicate that MC is a safe and effective complementary treatment for pain relief in patients with cancer. Our findings should be confirmed through randomised placebo-controlled trials.