Cancer pain (CP) is a prevalent condition with limited pharmacotherapeutic options. Cannabis-based medicinal products (CBMPs) have shown analgesic effects, but their efficacy in CP remains contentious. This study aims to evaluate the change in patient-reported outcome measures (PROMs) and adverse events (AEs) in CP patients treated with CBMPs. A case series was conducted using prospectively collected clinical data from the UK Medical Cannabis Registry. Primary outcomes were the changes in the Brief Pain Inventory (BPI), pain visual analogue scale (Pain-VAS), EQ-5D-5L, Generalized Anxiety Disorder-7 (GAD-7), Patient Global Impression of Change (PGIC) and Single-Item Sleep Quality Scale (SQS) questionnaires from baseline to 1, 3, and 6 months. AEs were recorded and graded. p < 0.050 was considered statistically significant. One hundred and sixty-eight participants were included. CBMPs were associated with improvements in all pain-specific PROMs at all follow-up periods (p < 0.050). Improvements in GAD-7, SQS, and EQ-5D-5L index scores were also observed (p < 0.050). Twenty-nine AEs (17.26%) were reported by five patients (2.98%), mostly mild-to-moderate (72.41%). Although the observational design means causality cannot be established, the findings support the development of future randomized controlled trials into CP management with CBMPs.