Muscle relaxants are used to reduce muscle spasms after muscle injury or for spasticity caused by chronic illnesses such as spinal cord injury. Skeletal muscle relaxants work on striated muscle, which attaches to the skeleton. As many as 35 percent of patients who visit a primary care physician for low back pain leave with a prescription for a muscle relaxant, reported Alec Meleger, M.D. of Spaulding Rehabilitation Hospital in 2006. Although several different classes of skeletal muscle relaxants are commonly prescribed, many have similar effects, especially on the central nervous system.
Central Nervous System Effects
Sedation--the most common effect of muscle relaxants, according to pain management physician Gerard Malanga, M.D--can be minimized by prescribing a muscle relaxant to be taken at bedtime. Dizziness, confusion, headache, blurred vision and nervousness have also been described by people taking any type of muscle relaxant. Seizures and hallucinations can occur in people taking baclofen, particularly if the drug is withdrawn suddenly, Meleger warns.
Cardiovascular Effects
Different muscle relaxants can cause cardiovascular effects. Effects of tizanidine include slow heartbeat, palpitations and low blood pressure. Botulinum toxin type A and B can cause irregular heartbeat, high blood pressure and heart attack, Meleger states. Heart failure and pericarditis, inflammation of the lining around the heart, occur rarely with dantrolene, especially if administered intrathecally into the spine, the Encyclopedia of Surgery warns.
Gastrointestinal Effects
Effects on the gastrointestinal tract include dry mouth and nausea, reported with use of many different muscle relaxants. Constipation, reported with use of dantrolene and orphenadrine, and abdominal pain and diarrhea, reported by people taking cyclobenzaprine, can also occur.
Respiratory Effects
Respiratory depression and pleural effusion, fluid between the lungs and the membrane that surrounds them, have been reported in people taking dantrolene. Sore throat and hoarseness can occur in people taking botulinum toxin.
Urinary Effects
Urinary retention occurs as a side effect of several different muscle relaxants. Methocarbamol, quinine and chlorzoxazone can cause urine to change color, a temporary condition that resolves after stopping the medication, the Encyclopedia of Surgery reports.
