eMedExpert Home > Compare Drugs > Skeletal Muscle Relaxers

• Brief history
• FDA approved indications and uses
• Mechanism of action
• Efficacy
• Adverse reactions and side effects
• Cost
• Key differences between drugs
• Conclusions
• Brief comparison table

This review examines the efficacy, side effects, and safety of three commonly prescribed skeletal muscle relaxants metaxalone (Skelaxin), cyclobenzaprine (Flexril), and carisoprodol (Soma).

Skeletal muscle relaxants are commonly used drugs prescribed for the treatment of muscle spasms and discomfort. Muscle relaxants are not really a class of drugs, but rather a group of different drugs that each has an overall sedative effect on the body.

Muscle relaxants treat both muscle spasm and spasticity. These drugs relieve muscle spasms due to low back pain, neck pain, fibromyalgia, tension headaches. They also relieve spasticity due to cerebral palsy, multiple sclerosis, spinal cord injury, or stroke.

• Carisoprodol (brand name: Soma) was approved by the FDA in 1959. Carisoprodol is a muscle relaxant that is used to treat muscle spasms and musculoskelatal pain. Carisoprodol is also a drug treatment option for fibromyalgia symptoms.
• Cyclobenzaprine was FDA approved on August 26, 1977. Cyclobenzaprine is distributed and marketed by McNeil under the brand name Flexeril. Flexeril is one of the strongest muscle relaxants. It has been shown to be beneficial in fibromyalgia, muscle spasms and musculoskelatal pain.
• Metaxalone was approved by FDA on August 13, 1962. This medication is manufactured by by King Pharmaceuticals under the brand name Skelaxin. Skelaxin is indicated as an adjunct to rest, physical therapy, and other measures for the relief of discomforts associated with acute, painful musculoskeletal conditions.

All three medications are indicated as an adjunct to rest, physical therapy, and other measures for the relief of discomforts associated with acute, painful musculoskeletal conditions.

Carisoprodol, cyclobenzaprine and metaxalone are not found to be effective in the treatment of spasticity associated with upper motor neuron syndromes.

The mechanisms of action of the medications in this class are widely varied and many are not thoroughly understood.

Carisoprodol: The actions of carisoprodol are related to a central nervous system (CNS) mechanism and not to a direct effect on skeletal muscle. Carisoprodol appears to interrupt neuronal communication within the reticular formation and spinal cord. CNS depression produces sedation, and the perception of pain could be altered. Some believe that most of the benefit seen with carisoprodol is secondary to a generalized sedative effect.

Cyclobenzaprine: Since cyclobenzaprine is closely similar to amitriptyline in chemical structure, some of its effects are similar to the tricyclic antidepressants. Cyclobenzaprine relieves muscle spasms through a central action, possibly at the brain stem level, with no direct action on the neuromuscular junction or the muscle involved. It is not a peripheral neuromuscular blocker. Treatment with the drug reduces pain and tenderness, and improves mobility.

Metaxalone: The mechanism of action of metaxalone has not been established. Metaxalone has no direct effect on the contractile mechanism of striated muscle, the motor end plate, or the nerve fiber. Its mode of action may be due to general central nervous system depression.

No studies have documented superior efficacy of one skeletal muscle relaxant over another.

All three drugs appear to have equal efficacy, but their side effects vary considerably.

Sedation is the most commonly reported adverse effect of muscle relaxants. These medications should be used with caution in people driving motor vehicles or operating heavy machinery. More absolute contraindications do exist to the use of carisoprodol and cyclobenzaprine. By initially prescribing muscle relaxants at bedtime, the physician might take advantage of their sedative effects and minimize daytime drowsiness.

While cyclobenzaprine may not share the dangerous cardiac and neurological potential of its close relatives the TCAs, it does share other properties, particularly confusion, lethargy, and anticholinergic side effects, and may have some toxicity in overdose and in combination with other substances.

Carisoprodol presents the most significant concern, due particularly to its potential for dependence and abuse. Several investigators have called for carisoprodol to be classified as a controlled substance. Carisoprodol is thought to carry an important risk for abuse because of its metabolism to meprobamate.

In the head to head trial of cyclobenzaprine and carisoprodol, dry mouth was more frequent with cyclobenzaprine (38% vs. 10%) and dizziness less frequent (8% vs. 26%). Withdrawal rates due to adverse events were equal (8%).

Metaxalone has the fewest reported side effects of any skeletal muscle relaxants and appears to be the safest.

Carisoprodol and cyclobenzaprine are available in generic versions. Metaxalone is available only as brand name medication Skelaxin. So treatment with Skelaxin is more expensive than with carisoprodol or cyclobenzaprine.

• Cyclobenzaprine has been evaluated in the most clinical trials and so has the most proof of being effective. Related to TCAs, use with caution in cardiac patients
• Carisoprodol can be addictive.
• Metaxalone is the least likely to cause drowsiness making it more compatible with day time use. Do not use in hepatic dysfunction or patients with history of drug-induced anemia.

The centrally acting skeletal muscle relaxants have been shown to be efficacious in the treatment of painful musculoskeletal disorders. Studies have indicated they may be more effective in combination with analgesics.

Without a clearly superior agent, side effect profiles become a significant consideration in the selection of medications to be prescribed in individual cases.

	Carisoprodol Soma (Wallace)	Cyclobenzaprine Flexeril (McNeil)	Metaxalone Skelaxin (King)
FDA approval date	April 09, 1959	August 26, 1977	August 13, 1962
Pharmaceutical Forms	350 mg tablets	5 mg, 10 mg tablets	800 mg tablets
FDA approved indications	relief of discomforts associated with acute, painful musculoskeletal conditions	relief of muscle spasm associated with acute, painful musculoskeletal conditions	relief of discomforts associated with acute, painful musculoskeletal conditions
Most common side effects	- drowsiness - dizziness - ataxia - tremor - agitation	- drowsiness - dry mouth - dizziness	- drowsiness - dizziness - headache
Less common side effects	- irritability - headache - depressive reactions - insomnia - nausea - vomiting - tachycardia	- atigue/tiredness - asthenia - nausea - constipation - dyspepsia - unpleasant taste - blurred vision - headache - nervousness - tachycardia - arrhythmia	- nervousness - nausea - vomiting - gastrointestinal upse
Onset/ Duration of action	Onset: 30 min Duration: 4-6 hours	Onset: 1 hour Duration: 12-24 hours	Onset: 30 min- 1 hour Duration: 4- 6 hours
Generic avalability	Yes	Yes	No

• 1. The Merck Manual of Medical Information. Mark H. Beers et al., eds. 2nd Home Edition. Whitehouse Station, NJ: Merck; 2003.

Published: May 05, 2007
Last updated: January 07, 2009