Efficacy of a low-dose regimen of cyclobenzaprine hydrochloride in acute skeletal muscle spasm: results of two placebo-controlled trials

Background: Cyclobenzaprine hydrochloride is a muscle relaxant that is effective in improving muscle spasm, reducing local pain and tenderness, and increasing range of motion in acute, painful musculoskeletal conditions. Sedation is the most common adverse event associated with its use at the usual dosage of 10 mg TID. Studies in healthy adults suggest that a lower dose may produce less sedation. Because cyclobenzaprine's duration of action is 4 to 6 hours, reducing the dosing frequency to 10 mg BID would create a potentially painful untreated interval between doses. The alternative is administration of a lower dose (eg, 5 or 2.5 mg) TID.

Objective: These studies were designed to assess the efficacy and tolerability of cyclobenzaprine 2.5, 5, and 10 mg TID compared with placebo in patients with acute musculoskeletal spasm.

Methods: In 2 randomized, double-blind, placebo-controlled, parallel-group trials conducted at primary care centers in the United States, adult patients with acute painful muscle spasm of the lumbar or cervical region were randomly assigned to receive treatment with 2.5, 5, or 10 mg cyclobenzaprine TID or placebo for 7 days (study 1: cyclobenzaprine 5 or 10 mg TID or placebo; study 2: cyclobenzaprine 2.5 or 5 mg TID or placebo). The primary efficacy measures were patient-rated clinical global impression of change, medication helpfulness, and relief from starting backache. Neither study included a nonsteroidal anti-inflammatory drug (NSAID) as an active control. Although physicians frequently prescribe an analgesic or NSAID in addition to cyclobenzaprine, these studies were not designed to assess whether adding cyclobenzaprine provides a benefit over that of an analgesic.

Results: One thousand four hundred five patients (737 study 1; 668 study 2), two thirds with low back pain and one third with neck pain, were randomized to treatment. Their mean age was 42 years, and approximately 89% were white. In both studies, patients receiving cyclobenzaprine 5 or 10 mg had significantly higher mean scores on the primary efficacy measures compared with those receiving placebo (study 1-P</=0.001 cyclobenzaprine 5 and 10 mg vs placebo, all measures at visits 2 and 3; study 2-P</=0.03 cyclobenzaprine 2.5 mg vs placebo, relief from starting backache on day 3 only; cyclobenzaprine 5 mg vs placebo, patient-rated clinical global impression of change, medication helpfulness, and relief from starting backache at visit 3 or day 7 only). On day 7, significantly more patients receiving cyclobenzaprine 5 or 10 mg reported relief compared with placebo recipients (P < 0.05 all cyclobenzaprine groups vs placebo). Onset of relief was apparent within 3 or 4 doses of the 5-mg regimen. In the subanalysis of the proportion of responders in the pooled 5-mg groups who did and did not report somnolence, a meaningful treatment effect was observed on all primary efficacy variables in patients who did not report somnolence, suggesting that efficacy was independent of sedation. Cyclobenzaprine was well tolerated. Somnolence and dry mouth, the most common adverse effects, were mild and dose related. Overall, >/= 1 adverse event was reported in 54.1%, 61.8%, and 35.4% of patients receiving cyclobenzaprine 5 or 10 mg or placebo, respectively, in study 1 and by 43.9%, 55.9%, and 35.4% of patients receiving cyclobenzaprine 2.5 or 5 mg or placebo, respectively, in study 2. Adverse events were the primary reason for discontinuation of treatment in the cyclobenzaprine 5- and 10-mg groups in both studies. In study 2, ineffectiveness of therapy was the main reason for discontinuation of therapy in the group receiving cyclobenzaprine 2.5 mg.

Conclusions: Cyclobenzaprine 2.5 mg TID was not significantly more effective than placebo. The cyclobenzaprine 5- and 10-mg TID regimens were associated with significantly higher mean efficacy scores compared with placebo. Cyclobenzaprine 5 mg TID was as effective as 10 mg TID, and was associated with a lower incidence of sedation.