Skeletal muscle relaxants are drugs that relax striated muscles (those that control the skeleton). They are a separate class of drugs from the muscle relaxant drugs used during intubations and surgery to reduce the need for anesthesia and facilitate intubation.
Skeletal muscle relaxants may be used for relief of spasticity in neuromuscular diseases such as multiple sclerosis, as well as for spinal cord injury and stroke. They may also be used for pain relief in minor strain injuries and control of the muscle symptoms of tetanus. Dantrolene (Dantrium) has been used to prevent or treat malignant hyperthermia in surgery.
The muscle relaxants are divided into two groups: centrally acting and peripherally acting. The centrally acting group appears to act on the central nervous system (CNS), and contains 10 drugs that are chemically different. Only dantrolene has a direct action at the level of the nerve-muscle connection.
Baclofen (Lioresal) may be administered orally or intrathecally (introduced into the space under the arachnoid membrane that covers the brain and spinal cord) for control of spasticity due to neuromuscular disease.
Several drugs, including carisoprodol (Soma), chlorphenesin (Maolate), chlorzoxazone (Paraflex), cyclobenzaprine (Flexeril), diazepam (Valium), metaxalone (Skelaxin), methocarbamol (Robaxin), and orphenadrine (Norflex), are used primarily as an adjunct for rest in management of acute muscle spasms associated with sprains. Muscle relaxation may also be an adjunct to physical therapy in rehabilitation following stroke, spinal cord injury, or other musculoskeletal conditions.
Diazepam and methocarbamol are also used by injection for relief of tetanus.
Dose varies with the drug, route of administration, and purpose. There may be individual variations in absorption that require doses higher than those usually recommended (particularly with methocarbamol). The consumer is advised to consult specific references or ask a doctor for further information.
All drugs in the muscle relaxant class may cause sedation. Baclofen, when administered intrathecally, may cause severe CNS depression with cardiovascular collapse and respiratory failure.
Diazepam may be addictive, and is a controlled substance under federal law.
Dantrolene has a potential for hepatotoxicity. The incidence of symptomatic hepatitis is dose related, but may occur even with a short period of doses at or above 800 mg per day, which greatly increases the risk of serious liver injury. Overt hepatitis has been most frequently observed between the third and twelfth months of therapy. Risk of liver injury appears to be greater in women, in patients over 35 years of age, and in patients taking other medications in addition to dantrolene.
Tizanidine may cause low blood pressure, but this may be controlled by starting with a low dose and increasing it gradually. Rarely, the drug may cause liver damage.
Methocarbamol and chlorzoxazone may cause harmless color changes in urine—orange or reddish purple with chlorzoxazone; and purple, brown, or green with methocarbamol. The urine will return to its normal color when the patient stops taking the medicine.
Most drugs in the muscle relaxant class are well tolerated, but not all of these drugs have been evaluated for safety in pregnancy and breastfeeding.
Baclofen is pregnancy category C. It has caused fetal abnormalities in rats at doses 13 times above the human dose. Baclofen passes into breast milk, so breastfeeding while taking baclofen is not recommended.
Diazepam is category D. All benzodiazepines cross the placenta. Although the drugs appear to be safe for use during the first trimester of pregnancy, use later in pregnancy may be associated with cleft lip and palate. Diazepam should not be taken while breastfeeding. It was found that infants who were breastfed while their mothers took diazepam were excessively sleepy and lethargic.
Dantrolene is category C. In animal studies, it has reduced the rate of survival of the newborn when given in doses seven times the normal human dose. Mothers should not breastfeed while receiving dantrolene.
Skeletal muscle relaxants have many potential drug interactions. It is recommended that individual references be consulted.
Because these drugs cause sedation, they should be used with caution when taken with other drugs that may also cause drowsiness.
The activity of diazepam may be increased by drugs that inhibit its metabolism in the liver. These include cimetidine, oral contraceptives, disulfiram, fluoxetine, isoniazid, ketoconazole, metoprolol, propoxyphene, propranolol, and valproic acid.
Dantrolene may have an interaction with estrogens. Although no interaction has been demonstrated, the rate of liver damage in women over the age of 35 who were taking estrogens is higher than in other groups.
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Samuel D. Uretsky, PharmD