How Do Skeletal Muscle Relaxants Work?

Skeletal muscle relaxants are medications used to relieve muscle spasms and pain caused by musculoskeletal disorders. Skeletal muscle relaxants work in various ways to relax the skeletal muscles. Some muscle relaxants work on the muscle cells, some work on nerve cells, (neurons) and some depress the central nervous system (CNS).

Skeletal muscles are attached to the skeleton by tendons and are responsible for voluntary movement. Motor neurons enable muscle contraction by releasing chemicals (neurotransmitters) that activate the muscles. In the reflex contraction of muscles, input from sensory (afferent) nerves activates the motor (efferent) nerves.

The abnormal firing of motor neurons due to certain disorders may result in muscle spasms and painful contractions. Some of the ways in which skeletal muscle relaxants work to relax skeletal muscles include the following:

• Inhibit afferent nerves from releasing excitatory neurotransmitters
• Inhibit the activity of interneurons which connect the afferent and efferent nerves, and prevent transmission of nerve signals
• Block the release of acetylcholine, the neurotransmitter that motor neurons release to make muscles contract
• Interfere with calcium release within the skeletal muscle cells and block their activity
• Increase the level of the neurotransmitter gamma aminobutyric acid (GABA), which inhibits neuronal activity
• Produce general central nervous system depression which relaxes muscles and induces sedation

Skeletal muscle relaxants may be administered through many routes such as:

• Oral: Tablets, solutions, capsules
• Injections:
  • Intravenous (IV): Injected into the vein
  • Intramuscular (IM): Injected into the muscle
  • Intrathecal: Injected into the cerebrospinal fluid in the spinal canal
• Rectal: Gel applied inside the rectum

Skeletal muscle relaxants may be used in the treatment of the following conditions:

• Muscle spasticity: Stiff and tight muscles caused by
  • Multiple sclerosis (MS)
  • Cerebral palsy
• Dystonia: Neurological disorder that causes involuntary repetitive muscle contractions
• Trigeminal neuralgia: A facial nerve disorder that causes sudden severe pain
• Complex regional pain syndrome: A chronic pain condition caused by dysfunction in the nervous system
• Musculoskeletal pain: Pain caused by musculoskeletal disorders
• Muscle spasms: Muscle cramps from involuntary muscle contractions
• Malignant hyperthermia: A condition that causes high body temperature and severe muscle contractions
• Neuroleptic malignant syndrome: A life-threatening disorder from reaction to antipsychotic drugs, which causes fever, muscle rigidity, and altered mental status
• Muscle pain: Muscle pain caused by conditions such as:
  • Duchenne muscular dystrophy
  • Phosphorylase deficiency
• Succinylcholine-induced fasciculations and post-operative muscle pain: Muscle twitches and pain induced by succinylcholine which is used during anesthesia 
• Wolfram syndrome: A genetic condition that affects many of the body’s systems including the neuromuscular system.
• Organophosphate exposure: A neurotoxic substance that can cause muscle weakness, cramps, and paralysis
• Systemic sclerosis: A systemic autoimmune connective tissue disease
• Tetanus: Painful contractions of neck and jaw muscles caused by a bacterial infection
• Parkinson’s disease: A progressive nervous system disorder that impairs movement

Diazepam is a skeletal muscle relaxant that is also an antianxiety and anticonvulsant agent, used in the treatment of:

• Anxiety
• Alcohol withdrawal
• Endoscopy
• Preoperative sedation
• ICU sedation
• Muscle spasm
• Seizure disorder
• Status epilepticus, a medical emergency when a seizure lasts longer than five minutes
• Repetitive seizures caused by Dravet syndrome, a rare type of epilepsy

Side effects of skeletal muscle relaxants may include:

• Drowsiness
• Dizziness
• Lightheadedness
• Asthenia (weakness)
• Nausea
• Vomiting
• Confusion
• Hypotonia (low muscle tone)
• Headache
• Insomnia
• Constipation
• Urinary frequency
• Urinary retention
• Urinary incontinence
• Dysuria (painful urination)
• Nocturia (frequent urination at night)
• Hematuria (blood in urine)
• Crystalluria (crystals in urine)
• Increased hemoglobin and red blood cells in urine
• Fatigue
• Malaise
• Pruritus (itching)
• Urticaria (hives)
• Rash
• Acneiform rash (raised acne-like rash)
• Eczema eruption
• Sloughing (shedding of skin)
• Immune hypersensitivity reactions
• Angioneurotic edema (swelling in the tissue under the skin or mucous membranes)
• Anaphylaxis (severe allergic reaction), rare
• Muscle pain
• Convulsion
• Paresthesia (prickling sensation)
• Accidental injury
• Speech disorder
• Confusion
• Nervousness
• Depression
• Euphoria
• Agitation
• Excitement
• Abnormal thinking
• Irritability
• Hallucinations
• Anxiety
• Tremor
• Suicidal ideation/attempt
• Epileptic seizure
• Rigidity
• Dystonia (involuntary muscle contractions)
• Dysautonomia (a disorder of the autonomic nervous system)
• Ataxia (coordination disorder)
• Coma
• Tinnitus (ringing in the ear)
• Slurred speech
• Dry mouth
• Peripheral edema
• Increased salivation
• Visual disturbances
• Amblyopia (lazy eye)
• Diplopia (double vision)
• Blurred vision
• Nystagmus (involuntary eye movement)
• Strabismus (crossed eyes)
• Miosis (pupil constriction)
• Mydriasis (pupil dilation)
• Increased intraocular pressure
• Diarrhea
• Anorexia (loss of appetite)
• Dysgeusia (taste disorder)
• Dysphagia (swallowing difficulties)
• Abdominal pain or cramps
• Epigastric distress
• Indigestion
• Adynamic ileus (paralysis of intestinal muscles)
• Gastrointestinal hemorrhage
• Occult blood in stool
• Hepatotoxicity (toxicity to the liver)
• Cholestasis (reduced bile flow)
• Elevated liver enzyme levels
• Hepatitis
• Jaundice
• Hypoventilation (shallow breathing/respiratory depression)
• Dyspnea (shortness of breath)
• Pleural effusion with associated eosinophilia
• Palpitations
• Chest pain
• Heart failure
• Arrhythmia (irregular heartbeat)
• Tachycardia (rapid heartbeat)
• Bradycardia (slow heartbeat)
• Hypertension (high blood pressure)
• Hypotension (low blood pressure)
• Erratic blood pressure
• Stroke
• Heart block
• Myocardial infarction (heart attack)
• Syncope (fainting)
• Vertigo
• Vasodilation
• Local effects from IV administration such as:
  • Pain and swelling
  • Phlebitis (inflammation of veins)
  • Thrombophlebitis (phlebitis with a blood clot in the vein)
  • Carpal tunnel syndrome
  • Tissue necrosis
• Low blood cell count conditions such as:
  • Anemia
  • Aplastic anemia (anemia due to reduced red cell production)
  • Hemolytic anemia (anemia from rapid destruction of red cells)
  • Leukopenia (low leukocyte level)
  • Neutropenia (low neutrophil level)
  • Thrombocytopenia (low platelet count)
• Lymphocytic lymphoma (a type of blood cancer)
• Back pain
• Pneumonia
• Impotence
• Difficulty in erection
• Inability to ejaculate
• Sexual dysfunction
• Ankle edema
• Excessive perspiration
• Facial flushing
• Fever
• Abnormal hair growth
• Nasal congestion
• Conjunctivitis (inflammation of the conjunctiva, the membrane over the whites of the eye and inner eyelid surfaces)
• Pharyngitis (inflammation of the pharynx)
• Weight gain
• Onset or aggravation of scoliosis (sideways curvature of the spine)
• Orthostatic hypotension (blood pressure drop when standing up from sitting or lying down)
• Serotonin syndrome
• Death

Information contained herein is not intended to cover all possible side effects, precautions, warnings, drug interactions, allergic reactions, or adverse effects. Check with your doctor or pharmacist to make sure these drugs do not cause any harm when you take them along with other medicines. Never stop taking your medication and never change your dose or frequency without consulting your doctor.

Generic and brand names of skeletal muscle relaxant drugs include:

• Amrix
• arbaclofen (pending FDA approval)
• baclofen
• carisoprodol
• chlorzoxazone
• cyclobenzaprine
• Dantrium
• dantrolene
• Diastat
• Diastat AcuDial
• diazepam
• Gablofen
• Lioresal
• metaxalone
• methocarbamol
• Ontinua ER (pending FDA approval)
• orphenadrine
• Ozobax
• Revonto
• Robaxin
• Ryanodex
• Skelaxin
• Soma
• Valium