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Cyclobenzaprine, initially marketed under the trade name Flexeril(TM), has been available for human use since 1977 [1]. Few publications identify cyclobenzaprine contemporaneously as evidenced by cyclobenzaprine being cited only ten times over the last 2 years in the PubMed scientific database. Nevertheless, the drug continues to enjoy modest use; in 2003, for example, approximately 300,000 prescriptions for generic cyclobenzaprine were made in Canada (population at the time: 31.5 million) [2]. Despite the latter, however, cyclobenzaprine has lost its place in the top 100 generic prescription drugs sold in Canada over the period of 2003-2007 [2,3]. Given its relative safety and tolerability, this drug will continue to be prescribed. The spectrum of its use deserves review and the potential for furthering of this spectrum remains.
General considerations
Early studies of cyclobenzaprine use assessed this drug for potential mood-altering properties, such as depression, and this was initially of promise, given that cyclobenzaprine differs from amitriptyline by only one double bond. Eventually, most subsequent studies focused on the drug's muscle relaxant properties. As such, this pharmacological agent was classified as an antispasmodic skeletal muscle relaxant with many properties akin to benzodiazepines, carisoprodol, metaxalone, chlorzoxazone, methocarbamol, tizanidine and orphenadrine [4]. The antispasmodic quality of cyclobenzaprine appears to apply best to focal muscular disturbances [5,6]. The pharmacological agents listed as potential muscle relaxants are many but some have chosen to distinguish those that give rise to an antispasmodic effect from those that have an antispastic effect [7,8]. The latter include agents such as dantrolene, baclofen or gabapentin. One study has particularly assessed cyclobenzaprine (60 mg/day in divided doses) and placebo in a double-blinded crossover trial for 2 weeks among patients with spasticity as a result of either CNS or PNS disorders; there was no significant difference [9].
Most assessments of cyclobenzaprine include three-times a day dosing, although some have used the drug as a single night-time dose owing to its sedative effect. A recently marketed oral, extended-release form (Amrix® ) in the USA has employed 15 or 30 mg units [10]. Muscle relaxant use in the USA has been reviewed recently and is considerable, as estimated by the findings in which the 1-month prevalence was 1% [11]. Muscle relaxants were more often used in the age group of...