Background: Muscle spasms often result in back pain, which imposes significant social and economic burden on patients and society (1).

Methods: Data were pooled from two identical double-blind, parallel-group, placebo-controlled studies in patients with muscle spasms associated with acute, painful musculoskeletal conditions (n=504). Patients were randomized to: cyclobenzaprine hydrochloride modified-release (CMR) 15mg (n=127) or 30mg (n=126) once daily, cyclobenzaprine immediate-release (CIR) 10mg 3x daily (n=123), or placebo (n=128), for 14 days. Primary outcome measures (Day 4 or after 3 days of treatment) were patient's rating of medication helpfulness and physician's clinical global assessment (5-point scales). Daytime drowsiness was included among the secondary outcomes. Patients provided written informed consent; study protocols were IRB approved.

Results: 330 (65.5%) patients completed 14 days of treatment. At Day 4, the distribution of responses for patient's rating of medication helpfulness was significantly different for CMR vs placebo groups (15mg, P=0.014; 30mg, P=0.002). 19.0% of patients in CMR 15mg and 23.3% in CMR 30mg groups had “very good” to “excellent” responses versus 14.8% for placebo and 20.3% for CIR. The distribution of responses also differed significantly for CMR 30mg vs placebo (P=0.015) at Day 8, and for CMR 15mg (P=0.016) and 30mg (P=0.014) vs placebo at Day 14. There were no differences for CMR vs placebo or CIR on the physician's clinical global assessment. Significantly more daytime drowsiness occurred with CMR > placebo (P≤0.002).  At Day 4, rates of “no drowsiness” or “very little drowsiness” were 50.4%, 42.1%, and 28.8% for the CMR 15mg, CMR 30mg, and CIR groups, respectively. Frequently reported adverse events included dry mouth, dizziness, and headache; 2 serious adverse events were reported (unrelated to CMR or CIR).

Conclusions: Once-daily CMR 15mg and 30mg alleviated muscle spasms with similar efficacy to CIR, but with less daytime drowsiness.