Skeletal Muscle Relaxants: High Risk of Side Effects with Questionable Efficacy


Skeletal muscle relaxants, or SMRs, are commonly prescribed in the ambulatory care setting for a variety of conditions, including lower back pain, fibromyalgia, multiple sclerosis and insomnia. While utilization of this class of drugs is widespread, clinical evidence to support this use is scant and further complicated by the high prevalence of potentially harmful central nervous system, or CNS, side effects. Concern over the widespread use of these agents in light of poor evidence, and almost certain CNS depressant side effects, has prompted the Centers for Medicare & Medicaid Services (CMS) to include SMRs on its list of high-risk drugs to avoid in older patients.

Compared to agents in other recognized medication classes such as statins, SSRIs or ACE inhibitors, SMRs are a largely heterogeneous group with varying mechanisms of action and pharmacologic profiles. Thus, the mechanisms by which these agents exert their muscle relaxant properties are often not fully understood. The class can, however, be described in terms of two categories of use: antispasmodic agents used for musculoskeletal conditions and antispastic agents used for CNS disease. Current guidelines for musculoskeletal pain recommend short-term use (approximately two weeks) of SMRs as second-line or adjunct agents, while guidelines for use in CNS diseases such as multiple sclerosis also recommend SMRs but stress limited efficacy data. All guidelines also warn about the high prevalence of CNS-related side effects, most notably drowsiness and sedation.

Two systemic reviews of SMR trials reinforce guideline cautions. These reviews stress poor clinical trial design, short study duration (between seven and 14 days), marginal effect size and high incidence of CNS depressant effects as reasons for judicious use of SMRs. Cyclobenzaprine is the most heavily studied of the group and shows modest efficacy for musculoskeletal conditions. Baclofen, tizanidine and dantrolene are the agents of choice for spasticity in CNS disease. The use of carisoprodol (brand name Soma) also is discouraged due to its potential to cause physical and psychological dependence. Finally, no comparative data shows favorable efficacy or safety for one agent over another.

With limited evidence for efficacy and strong evidence for side effects, SMRs should be reserved as alternative or adjunct to first-line agents, and should be avoided in elderly patients, especially for chronic musculoskeletal conditions. Multimodal approaches should be employed to prevent regular use of SMRs, as there is no evidence that these medications are beneficial when used chronically. Patients being prescribed SMRs – in particular, older patients – should be monitored carefully for side effects and informed about therapy expectations.

Resources

  1. Chou R, Peterson K, Helfand M. Comparative efficacy and safety of skeletal muscle relaxants for spasticity and musculoskeletal conditions: a systematic review. J Pain Symptom Manage. 2004; 28 (2): 140-175.
  2. Van Tulder MW, Touray T, Furlan AD, Solway S, Bouter LM. Muscle relaxants for non-specific low back pain. Cochrane Database Syst Rev. 2003; (2).