Comparison of the efficacy and safety of acetaminophen versus NSAIDs for the treatment of chronic pain in older adults with osteoarthritis of the hip and knee: Findings from the randomized, double-blind, parallel-group, non-inferiority RETHINK study

Objective: This study aimed to evaluate the non-inferiority of acetaminophen compared with non-steroidal anti-inflammatory drugs (NSAIDs) for the treatment of chronic osteoarthritis-related pain in older adults. Design: This multicenter, randomized, double-blind, parallel-group study enrolled patients aged 65 years or older with osteoarthritis-related pain. Participants were randomly assigned to receive acetaminophen (1800 mg/day) or NSAIDs (loxoprofen 180 mg/day or celecoxib 200 mg/day). The primary endpoint was the change in Brief Pain Inventory (BPI) item 3 (worst pain) score from baseline to week 8. The secondary endpoints included the change in BPI item 3 score from baseline to week 4, quality of life, gastrointestinal disorders, and renal and liver function parameters. Results: Of the 400 patients enrolled, 191 and 197 were in the acetaminophen (mean age 73.6 years; 83.2% female) and NSAID groups (mean age 73.3 years; 74.6% female), respectively. The least-squares mean change in BPI item 3 scores at 8 weeks was -1.79 in the acetaminophen group and -1.94 in the NSAIDs group. The between-group difference in BPI item 3 scores change was 0.14 (95% CI, -0.33 to 0.61). No major safety concerns were identified; however, gastrointestinal disorders occurred more frequently with NSAIDs and were the most common cause of treatment discontinuation. Conclusions: In this RETHINK study, acetaminophen achieved a similar reduction in osteoarthritis-related pain to NSAIDs in older adults after eight weeks; however, non-inferiority was not demonstrated. In terms of adverse events, acetaminophen was associated with fewer gastrointestinal disorders. These findings suggest that treatment choice may depend on the balance between analgesic efficacy and safety considerations. Trial registration number: The study is registered in the Japan Registry of Clinical Trials (jRCTs071200112).