Recommendations for selecting muscle relaxants for musculoskeletal conditions such as low back pain, fibromyalgia, tension headaches, and myofascial pain syndrome are reviewed in an article published in the August 1 issue of American Family Physician.

"Prescription rates for nonspecific back pain revealed that skeletal muscle relaxants accounted for 18.5 percent of prescriptions compared with 16.3 percent for NSAIDs [nonsteroidal anti-inflammatory drugs] and 10 percent for cyclooxygenase-2 inhibitors," write Sharon See, PharmD, BCPS, and Regina Ginzburg, PharmD, from St. John\'s University College of Pharmacy and Allied Health Professions in Jamaica, New York. "Because of the high rate of prescribing skeletal muscle relaxants, an understanding of the risks and benefits of this class of drugs is vital. This article presents evidence regarding the use of antispasmodic skeletal muscle relaxants for various musculoskeletal conditions, and appropriate drug selection if a skeletal muscle relaxant is required."

Despite the widespread use of skeletal muscle relaxants for treatment of musculoskeletal conditions, data supporting this practice come primarily from studies with methodologic limitations. Treatment goals for low back or neck pain, fibromyalgia, tension headaches, and myofascial pain syndrome include alleviation of muscle pain and improvement in functional ability allowing return to work or to customary activities.

Skeletal muscle relaxants have not been shown to be superior to acetaminophen or NSAIDs for low back pain, although systematic reviews and meta-analyses support short-term use of skeletal muscle relaxants for relief of acute low back pain in patients who do not respond to or tolerate NSAIDs or acetaminophen. Existing recommendations for treating tension headaches are similar.

There are 2 main categories of skeletal muscle relaxants: antispastic (such as baclofen or dantrolene) for conditions such as cerebral palsy and multiple sclerosis and antispasmodic agents for musculoskeletal conditions. Evidence is extremely limited to support the use of antispastic agents for musculoskeletal conditions, for which an antispasmodic agent is typically more appropriate.

The most commonly prescribed antispasmodic agents are carisoprodol, cyclobenzaprine, metaxalone, and methocarbamol.

A meta-analysis of cyclobenzaprine showed that it was superior to placebo for treating fibromyalgia but that it was not as effective as antidepressants. Recent guidelines regarding treatment of fibromyalgia recommend a comprehensive approach using tramadol, antidepressants, and/or a heated pool, with or without exercise.

In comparison trials, no single skeletal muscle relaxant has been established superior to another. The most widely studied agent is cyclobenzaprine, with demonstrated efficacy for various musculoskeletal conditions but with significant sedation. Tizanidine, which also causes marked sedation or cyclobenzaprine, may be useful in patients with insomnia because of severe muscle spasms.

All skeletal muscle relaxants are associated with adverse effects including dizziness and drowsiness, and the potential for these adverse effects should be clearly explained to the patient. Although methocarbamol and metaxalone are less sedating than tizanidine and cyclobenzaprine, evidence is limited for their efficacy. A specific skeletal muscle relaxant should be chosen according to consideration of adverse effect profile, patient preference, potential for abuse, and possible interactions with other prescribed medications because comparable efficacy data are limited.

Carisoprodol is typically prescribed at a dose of 350 mg 4 times daily, but it is not recommended for children younger than 12 years. Adverse events include dizziness, drowsiness, headache, rare idiosyncratic reactions (mental status changes, transient quadriplegia, and temporary loss of vision) after the first dose that may require hospitalization and allergy-type reactions that may occur after the first to fourth dose. Physical or psychological dependence may occur, and withdrawal symptoms may occur when carisoprodol is discontinued. When combined with benzodiazepines, barbiturates, codeine or its derivatives, or other muscle relaxants, carisoprodol may contribute to respiratory depression.

Starting dose for cyclobenzaprine is 5 mg 3 times daily, which may be increased to 10 mg 3 times daily. Adverse effects include anticholinergic effects of drowsiness, dry mouth, urinary retention, and increased intraocular pressure. Rare but serious adverse events may include arrhythmias, seizures, or myocardial infarction. It should be avoided in older patients and in patients with glaucoma, and there may be possible drug interactions with cytochrome P450 inhibitors.

Metaxalone is prescribed at 800 mg 3 to 4 times daily and is not recommended in children younger than 12 years. Drowsiness, dizziness, headache, and nervousness are common adverse effects. Rare adverse events may include leukopenia or hemolytic anemia; elevation in liver function tests; or nausea, vomiting, and diarrhea. Paradoxic muscle cramps may also occur.

Methocarbamol is given at a dosage of 1500 mg 4 times daily for the first 2 to 3 days and is then reduced to 750 mg 4 times daily. Use of this drug may be associated with black, brown, or green urine; mental status impairment; or possible exacerbation of myasthenia gravis symptoms.

Specific clinical recommendations for practice, and their accompanying level of evidence rating, are as follows:

• Skeletal muscle relaxants are not considered first-line therapy for musculoskeletal conditions (level of evidence, C).
• For acute low back pain, skeletal muscle relaxants may be used as adjunctive therapy (level of evidence, B).
• For acute low back pain, antispasmodic agents should be used short term (2 weeks; level of evidence, C).
• Evidence to date does not clearly support the superiority of 1 skeletal muscle relaxant to another for musculoskeletal spasms (level of evidence, B).
• Specific drug profile and individual patient situation should guide the choice of skeletal muscle relaxant (level of evidence, C).