Prescription Muscle Relaxers
Muscle relaxants are widely used in the treatment of musculoskeletal conditions. Carisoprodol, cyclobenzaprine, and metaxalone are the most commonly prescribed medications for the management of musculoskeletal pain1.
All SMRs do not share the exact same mechanism of action or molecular structure. In fact, they are structurally unrelated and have different pharmacokinetic properties.
However, all medications from this class target the brain, not the muscles, and have an insignificant, if any effect, at the muscle fiber level.
There are two categories of muscle relaxants: antispastic and antispasmodic agents. Antispastics are primarily used for nerve-related disorders. Antispasmodics are used to treat musculoskeletal conditions, characterized by muscle spasms.
The use of muscle relaxants continues to be a source of controversy among doctors, mainly because of their side effects. However, about 91% of physicians report using muscle relaxants.2
This monograph presents a better understanding of the risks and benefits of most commonly used medications in this class.
Skeletal muscle relaxants (SMR) include:
Carisoprodol is a centrally acting skeletal muscle relaxant. It is metabolized to meprobamate, a schedule III controlled substance with a potential for dependence. Although carisoprodol is not a controlled substance, it has significant potential for physical or psychological dependence.
Side effects include dizziness, drowsiness, headache. Withdrawal symptoms may occur when carisoprodol is discontinued. When combined with benzodiazepines, pain killers, or other muscle relaxers, it may contribute to respiratory depression.
Carisoprodol is available as generic, and in various brand forms such as Soma, Vanadom.
Cyclobenzaprine is the most well studied muscle relaxant and has been shown to be effective for various musculoskeletal conditions. Its chemical structure is similar to that of the tricyclics. Therefore it has similar pharmacologic effects.
The sedative properties of cyclobenzaprine may benefit persons with insomnia caused by severe muscle spasms. Unlike other relaxers, cyclobenzaprine can be used on a longer-term basis.
Side effects include drowsiness, sedation, dry mouth, urinary retention, and increased intraocular pressure. It should be avoided in older patients and in persons with glaucoma.
The exact mechanism of action of metaxalone is not fully understood, but its effect is presumed to be due to general depression of the central nervous system. It has no direct effect on the contractile mechanism of striated muscle, the motor endplate, or the nerve fiber. The advantages of metaxalone are low sedation, lack of abuse potential, and limited accumulation because of its short elimination half-life.
Common side effects include drowsiness, dizziness, headache, and nervousness. Paradoxic muscle cramps may also occur. Metaxalone may cause false-positive Benedict's tests.
Treatment with metaxalone may be expensive, as it is only available in brand form Skelaxin.
Methocarbamol is a centrally acting muscle relaxant that suppresses spinal polysynaptic reflexes and has no direct effect on skeletal muscles.
Initially for the first 2 to 3 days, methocarbamol can be taken four times a day at a dosage of 1500 mg. For maintenance therapy, 1500 mg can be given three times per day.
Frequent side effects include drowsiness, dizziness, nausea, anorexia, headache, blurred vision, muscular discoordination. Methocarbamol may cause black, brown, or green urine.
Tizanidine is a centrally acting alpha-2-agonist that exerts its antispastic effect by causing presynaptic inhibition of motor neuron hyperactivity. It has a relatively quick onset of effect of 1-2 hours and duration of only 3-6 hours. For this reason, tizanidine should be reserved for time-dependent activities.
Tizanidine should be prescribed through a gradual upward titration from an initial dose of 2 to 4 mg at bedtime up to the maximum of 8 mg three times per day. Tizanidine bedtime dose can provide an analgesic effect and improve sleep.
Orphenadrine is structurally related to diphenhydramine and has stronger anticholinergic and weaker sedative properties. It does not produce any direct effect on skeletal muscle, and its exact mode of action is unknown.
Orphenadrine has relatively long half-life, and the usual adult dosage is 100 mg twice per day.
Common side effects include drowsiness, dizziness, agitation, hallucinations, dry mouth, nausea, constipation, urinary retention.
Last updated: March, 2013