The aim of this study was to assess changes in validated patient-reported outcome measures after initiation of cannabis-based medicinal products (CBMPs) and the safety of CBMPs in patients with inflammatory arthritis. A prospective case series from the UK Medical Cannabis Registry was analyzed. The primary outcomes changes were in Brief Pain Inventory, McGill Pain Questionnaire, EuroQol 5-dimension 5-level (EQ-5D-5L), Generalised Anxiety Disorder-7 questionnaire, and Single-Item Sleep Quality Scale at 1, 3, 6, and 12 months of follow-up compared with baseline. Adverse events were analyzed in accordance with Common Terminology Criteria for Adverse Events, v.4.0. Statistical significance was defined as a P-value less than 0.050. Eighty-two patients met the inclusion criteria. Initiation of CBMP treatment was associated with improvements in Brief Pain Inventory, McGill Pain Questionnaire, EQ-5D-5L, Generalised Anxiety Disorder-7 questionnaire, and Single-Item Sleep Quality Scale at 1, 3, 6, and 12 months compared with baseline (P < 0.050). There were 102 (44.35%) mild adverse events, 97 (42.17%) moderate adverse events, and 31 (13.48%) severe adverse events recorded by 21 (25.61%) participants. This study suggests that CBMP treatment is associated with pain improvement and increased health-related quality of life for inflammatory arthritis patients. While causality cannot be inferred in this observational study, the results support the development of randomized control trials for inflammatory arthritis pain management with CBMPs.