III. Contraindications

IV. Efficacy

  1. More effective than Placebo, but less effective than first-line agents such as NSAIDs
  2. "Muscle relaxants" do not directly relax Muscles (most of effect is likely sedation)

V. Precautions

  1. Maximize Analgesics (NSAIDs and Acetaminophen) as they are generally more effective
  2. Dose as needed and primarily at night to assist sleep (primary effect is sedation)
  3. Limit to adjunctive (primarily bedtime use) in Acute Low Back Pain (and avoid in chronic Low Back Pain)
  4. Avoid concurrent use of CNS Depressants (e.g. Alcohol, Benzodiazepines, Opioids)
    1. Risk of respiratory depression and death

VI. Agents: Preferred

  1. Cyclobenzaprine (Flexeril) 5 mg (up to 10 mg) orally at bedtime up to three times daily
    1. Best studied of all antispasmodic agents, appears consistently effective, and is inexpensive
    2. Anticholinergic adverse effects (related to Tricyclic Antidepressants)
  2. Methocarbamol (Robaxin) 750 to 1500 mg four times daily
    1. Significantly less expensive than Flexeril
    2. Warn about Urine Color change

VII. Agents: Higher risk of Adverse Effects

  1. Chlorzoxazone (Parafon Forte) 250-750 mg 3-4 times daily
    1. Risk of liver abnormalities and GI upset
  2. Metaxalone (Skelaxin) 800 mg 3-4 times daily
    1. Risk of liver, hematologic abnormalities and GI upset
    2. Less effective and expensive
  3. Orphenadrine (Norflex) 100 mg twice daily
    1. Risk of Tachycardia, hematologic abnormalities and GI upset
  4. Orphenadrine with 770 mg Aspirin and 60 mg Caffeine (Orphengesic Forte)
    1. Very expensive ($25/tab) rebranding of an older Muscle relaxant
    2. Unclear if any Muscle relaxant is indicated (low efficacy) and risk of falls and Fractures in older adults
    3. Aspirin adds additional bleeding risk including Gastrointestinal Bleeding
    4. (2020) presc lett 27(9): 51

VIII. Agents: Higher risk of dependence

  1. Carisoprodol (Soma) 350 mg up to four times daily
    1. Schedule IV drug in U.S. as of January 2012
    2. Risk of transient neurologic changes, serious Allergic Reactions, Substance Abuse, and fatal Overdoses
    3. Metabolized to Meprobamate, a Sedative and FDA controlled substance similar to Barbiturates
    4. See Carisoprodol Poisoning
  2. Benzodiazepines
    1. Diazepam (Valium) 2-10 mg orally 3-4 times daily

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Related Studies

Cost: Medications

methocarbamol (on 5/17/2017 at Medicaid.Gov Survey of pharmacy drug pricing)
METHOCARBAMOL 500 MG TABLET Generic $0.07 each
METHOCARBAMOL 750 MG TABLET Generic $0.08 each
chlorzoxazone (on 5/17/2017 at Medicaid.Gov Survey of pharmacy drug pricing)
CHLORZOXAZONE 500 MG TABLET Generic $0.21 each
metaxalone (on 5/17/2017 at Medicaid.Gov Survey of pharmacy drug pricing)
METAXALONE 400 MG TABLET Generic $4.76 each
METAXALONE 800 MG TABLET Generic $2.35 each
skelaxin (on 6/1/2017 at Medicaid.Gov Survey of pharmacy drug pricing)
SKELAXIN 800 MG TABLET Generic $2.35 each
orphenadrine (on 5/17/2017 at Medicaid.Gov Survey of pharmacy drug pricing)
ORPHENADRINE ER 100 MG TABLET Generic $0.37 each

Ontology: Chlorzoxazone (C0008296)

Definition (NCI) A benzoxazolone derivative with mild sedative and centrally-acting muscle relaxant activities. Although its exact mechanism of action is unknown, chlorzoxazone (CZ) appears to act at the spinal cord and subcortical levels of the brain to inhibit multisynaptic reflex arcs involved in producing and maintaining muscle spasms. This agent is extensively hydroxylated by cytochrome P450 2E1 (CYP2E1) to 6-hydroxychlorzoxazone (HCZ),11,12 which is subsequently glucuronidated and eliminated renally. Highly selective for CYP2E1, CZ may be used as a selective probe for phenotyping CYP2E1 in humans; the ratio of HCZ-to-CZ plasma concentrations obtained 2 to 4 hours after oral administration of CZ may be used as a phenotypic measure of CYP2E1 enzymatic activity.
Definition (MSH) A centrally acting central muscle relaxant with sedative properties. It is claimed to inhibit muscle spasm by exerting an effect primarily at the level of the spinal cord and subcortical areas of the brain. (From Martindale, The Extra Pharmacopoea, 30th ed, p1202)
Concepts Pharmacologic Substance (T121) , Organic Chemical (T109)
MSH D002753
SnomedCT 400361007, 44908000
LNC LP18103-9, MTHU003391
English Chlorzoxazone, 2(3H)-Benzoxazolone, 5-chloro-, chlorzoxazone, muscle relaxants skeletal chlorzoxazone, chlorzoxazone (medication), Chlorzoxazone [Chemical/Ingredient], CHLORZOXAZONE, chlorzoxozone, Chlorzoxozone, Chlorzoxozone (product), Chlorzoxozone (substance)
Swedish Klorzoxazon
Czech chlorzoxazon
Finnish Klooritsoksatsoni
Japanese クロルゾキサゾン
Spanish clorzoxozona (producto), clorzoxazona (producto), clorzoxazona, clorzoxozona (sustancia), clorzoxozona, Clorzoxazona
Polish Chlorzoksazon
French Chlorzoxazone
German Chlorzoxazon
Italian Clorozossazone
Portuguese Clorzoxazona

Ontology: Methocarbamol (C0025659)

Definition (NCI) A carbamate with centrally acting muscle relaxant properties. Though the exact mechanism of action of methocarbamol was not established, it's postulated to be via a mechanism similar of carbamate, inhibition of acetylcholinesterase at synapses in the autonomic nervous system, neuromuscular junction, and central nervous system. Methocarbamol has no direct effect on the contractile mechanism of striated muscle, the motor end plate or the nerve fiber.
Definition (MSH) A centrally acting muscle relaxant whose mode of action has not been established. It is used as an adjunct in the symptomatic treatment of musculoskeletal conditions associated with painful muscle spasm. (From Martindale, The Extra Pharmacopoeia, 30th ed, p1206)
Concepts Pharmacologic Substance (T121) , Organic Chemical (T109)
MSH D008721
SnomedCT 387486000, 83973001
LNC LP16197-3, MTHU007392
English Carbamate, Guaiphenesin, Guaiphenesin Carbamate, Methocarbamol, 1,2-Propanediol, 3-(2-methoxyphenoxy)-, 1-carbamate, methocarbamol, methocarbamol (medication), muscle relaxants skeletal methocarbamol, METHOCARBAMOL, Methocarbamol [Chemical/Ingredient], Methocarbamol (product), Methocarbamol (substance)
Swedish Metokarbamol
Czech methokarbamol
Finnish Metokarbamoli
Japanese メトカルバモル, ロバキシン, メトカルバモール, グアイフェネシンカルバメート
Polish Robaksyna, Metokarbamol
Spanish metocarbamol (producto), metocarbamol (sustancia), metocarbamol, Carbamato de Guaifenesina, Metocarbamol
French Méthocarbamol
German Guaiphenesincarbamat, Methocarbamol
Italian Metocarbammolo
Portuguese Carbamato de Guaifenesina, Metocarbamol

Ontology: Orphenadrine (C0029309)

Definition (NCI) A muscarinic antagonist with muscle relaxant activity. Although the mechanism of action has not been determined, orphenadrine appears to block muscarinic acetylcholine receptors as well as N-methyl-D-aspartate (NMDA) receptors in the central nervous system (CNS). This may interfere with the transmission of nerve impulses from the spinal cord to the muscles and may help relax muscles. In addition, orphenadrine exerts antagonistic effects on histamine H1 receptors.
Definition (MSH) A muscarinic antagonist used to treat drug-induced parkinsonism and to relieve pain from muscle spasm.
Concepts Pharmacologic Substance (T121) , Organic Chemical (T109)
MSH D009966
SnomedCT 35035001, 372714007
LNC LP15175-0, MTHU002564
English Methyldiphenylhydramine, Orphenadrine, Ethanamine, N,N-dimethyl-2-((2-methylphenyl)phenylmethoxy)-, orphenadrine (medication), Orphenadrine [Chemical/Ingredient], ORPHENADRINE, orphenadrine, Orphenadrine (product), Orphenadrine (substance)
Swedish Orfenadrin
Czech orfenadrin
Finnish Orfenadriini
Japanese オルフェナドリン, メチルジフェニルヒドラミン
Polish Orfenandryna, Metylodifenylohydramina
Spanish orfenadrina (producto), orfenadrina (sustancia), orfenadrina, Metildifenilhidramina, Orfenadrina
French Orphénadrine
German Methyldiphenylhydramin, Orphenadrin
Italian Orfenadrina
Portuguese Metildifenilidramina, Orfenadrina

Ontology: skeletal muscle relaxants (C0037250)

Concepts Pharmacologic Substance (T121)
MSH D009465
SnomedCT 17893001, 372666009
English Muscle Relaxants, Skeletal, Relaxants, Skeletal Muscle, SKELETAL MUSCLE RELAXANTS, skeletal muscle relaxants (medication), muscle relaxants skeletal, [MS200] SKELETAL MUSCLE RELAXANTS, skeletal muscle relaxant, skeletal muscle relaxants, Skeletal muscle relaxant, Skeletal muscle relaxant (product), Skeletal muscle relaxant (substance), Skeletal muscle tone depressant, Skeletal muscle relaxant, NOS, Skeletal muscle tone depressant, NOS, Skeletal Muscle Relaxants
French Myorelaxants, Décontracturants des muscles striés, Relaxants musculaires, Myorésolutifs, Décontracturants, Décontracturants musculaires, Relaxants des muscles squelettiques
Czech myorelaxancia
Spanish Relajantes del Musculo Esquelético, depresor del tono muscular esquelético, miorrelajante esquelético, relajante de músculo esquelético (sustancia), relajante de músculo esquelético, relajante del músculo esquelético (producto), relajante del músculo esquelético (sustancia), relajante del músculo esquelético
German Skelettmuskelrelaxanzien
Portuguese Relaxantes de Musculo Esquelético
Italian Rilassanti muscolo scheletrici

Ontology: metaxalone (C0163055)

Definition (NCI) An oxazolidinone with centrally-acting skeletal muscle relaxant properties. Although the exact mechanism through which metaxalone exerts its effect is largely unknown, it might be due to general central nervous system (CNS) depression. Metaxalone has no direct effect on the contractile mechanisms of striated muscle, the motor end plate, or the nerve fiber.
Concepts Pharmacologic Substance (T121) , Organic Chemical (T109)
MSH C011301
SnomedCT 87599003, 404863006
LNC LP100044-9, MTHU035865
English 5-((3,5-dimethylphenoxy)methyl)-2-oxazolidinone, metaxalone, Zorane, 2-Oxazolidinone, 5-((3,5-dimethylphenoxy)methyl)-, metaxalone (medication), muscle relaxants skeletal metaxalone, METAXALONE, metaxalone [Chemical/Ingredient], Metaxalone, Metaxalone (substance), Metaxalone (product)
Spanish metaxalona (sustancia), metaxalona, metaxalona (producto)