Chlorzoxazone falls short in chronic postoperative pain management

The postoperative administration of chlorzoxazone does not appear to be useful in the management of chronic pain following total knee or hip replacement (TKR or THR), according to a study. 

The trial randomized 393 patients scheduled for TKR or THR to receive either 250-mg chlorzoxazone three times daily or placebo for the first 7 days after the surgery.  

The primary outcome was pain after a 5-meter walk evaluated 24 hours postoperative. Secondary outcomes included changes in preoperative pain at rest, worst pain in the last 24 hours, and Oxford Knee or Hip Score compared with 12 months follow-up. Adverse events were evaluated in the perioperative period. 

There were no significant between-group differences observed in any of the outcome parameters examined after TKR or THR. Neither chlorzoxazone nor placebo produced analgesic effects in patients who underwent TKR or THR, as indicated in pain after 5-meter walk 24-hours postsurgery (p>0.313), in any of the secondary outcomes (p>0.288), or adverse event (p>0.112). 

The muscle relaxant chlorzoxazone is administered for musculoskeletal pain and as an analgesic adjunct for postoperative pain. It acts by inhibiting multisynaptic reflex arcs implicated in producing and maintaining skeletal muscle spasm at the level of the spinal cord and subcortical areas of the brain. [J Pain Symptom Manage 2004;28:140-175; Acta Anaesthesiologica Scandinavica 2016;doi:10.1111/aas.12754]