Cyclobenzaprine HCL ER (extended-release) is a centrally acting skeletal muscle relaxant mainly indicated as an adjunct to rest and physical therapy for relief of muscle spasm associated with acute, painful musculoskeletal conditions. Its mechanism involves CNS depression and blockade of serotonin receptors, helping to relax muscles and reduce muscle stiffness without interfering with overall muscle function. The extended-release formulation offers consistent plasma concentration and improved convenience compared to immediate-release forms, with clinical evidence supporting effectiveness for conditions like muscle spasms and, occasionally, fibromyalgia pain relief.