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Class: Centrally Acting Skeletal Muscle Relaxants
VA Class: MS200
CAS Number: 95-25-0
Brands: Lorzone

Medically reviewed by Last updated on March 2, 2020.


Centrally acting skeletal muscle relaxant.b

Uses for Chlorzoxazone

Muscular Conditions

Adjunct to rest, physical therapy, analgesics, and other measures for the relief of discomfort associated with acute, painful musculoskeletal conditions.1 18

If pharmacologic therapy is required for acute low back pain (usually a benign and self-limiting condition105 106 108 ), experts state that an NSAIA or skeletal muscle relaxant may be considered.109 Skeletal muscle relaxants may provide small improvements in pain relief, but are associated with a high incidence of adverse effects (e.g., CNS effects).104 106 107 108 109 Use with caution after weighing risks against benefits.104 106 107 108

Various skeletal muscle relaxants appear to have comparable efficacy for low back pain relief.103 104 106 108

Chlorzoxazone is ineffective in the treatment of skeletal muscle hyperactivity secondary to chronic neurologic disorders (e.g., cerebral palsy) and other dyskinesias.b

Chlorzoxazone Dosage and Administration


Oral Administration

Administer orally.1 100 102



Muscular Conditions

Initial dosage for painful musculoskeletal conditions is 500 mg 3–4 times daily;1 102 if response inadequate, may increase dosage to 750 mg 3–4 times daily.

When desired response is obtained, reduce dosage to lowest effective level.1 100 102

Usual adult dosage: 250–500 mg 3–4 times daily.1 100 102

Cautions for Chlorzoxazone


  • Known intolerance to chlorzoxazone.1 100 102




Serious, possibly fatal, hepatotoxicity reported rarely; may be associated with increased serum AST, ALT, alkaline phosphatase, and bilirubin concentrations.1 101 102 Mechanism unknown but appears idiosyncratic and unpredictable.1

Perform liver function tests periodically in patients receiving long-term therapy.101

If elevated liver function tests or bilirubin reported or if signs and symptoms of hepatic dysfunction (e.g., fever, rash, anorexia, nausea/vomiting, fatigue, right upper quadrant pain, dark urine, or jaundice) occur, discontinue chlorzoxazone immediately.1

CNS Effects

Performance of activities requiring mental alertness or physical coordination may be impaired.b

Concurrent use of other CNS depressants may potentiate CNS depression.1 (See Specific Drugs under Interactions.)

Sensitivity Reactions

Allergic-type reactions reported; angioedema and anaphylactic reactions are extremely rare.1 Use with caution in patients with allergies or with a history of allergic drug reactions.1

Specific Populations


Safety during pregnancy not established; use in women of childbearing potential only when potential benefits outweigh potential risks.1


Not known whether chlorzoxazone is distributed into milk.b

Geriatric Use

Because of risk of injury, skeletal muscle relaxants should generally be avoided in geriatric patients.111

Common Adverse Effects

Drowsiness, dizziness.b

Interactions for Chlorzoxazone

Specific Drugs




CNS depressants (e.g., alcohol, barbiturates, opiate agonists)

Potential additive CNS depression1 b

Use concomitantly with cautionb

Chlorzoxazone Pharmacokinetics


Well absorbed following oral administration, with peak plasma concentrations usually attained within 1–2 hours.1 24


Usually within 1 hour.b


3–4 hours.b



Not known whether chlorzoxazone crosses the placenta or is distributed into milk.b



Rapidly metabolized in the liver to 6-hydroxychlorzoxazone, an inactive metabolite.1 24

Elimination Route

Excreted principally in urine as a conjugated glucuronide; <1% excreted as unchanged drug.1 24


66 minutes.b





20–25°C.1 100 102


  • CNS depressant with sedative and skeletal muscle relaxant effects.b

  • Inhibits multisynaptic reflex arcs at the level of the spinal cord and subcortical areas of the brain; results in reduction of skeletal muscle spasm with relief of pain and increased mobility of involved muscles.1

Advice to Patients

  • Importance of informing clinician if any early signs or symptoms of possible liver dysfunction (e.g., fatigue, anorexia, nausea and/or vomiting, fever, rash, jaundice, dark urine, right upper quadrant pain) occur.1

  • Potential to impair mental alertness or physical coordination, especially with concomitant use of alcohol or other CNS depressants; use caution when driving or operating machinery.1 b

  • Importance of informing clinicians of existing or contemplated concomitant therapy, including prescription and OTC drugs and dietary or herbal supplements, as well as concomitant illnesses.1

  • Importance of women informing clinicians if they are or plan to become pregnant or plan to breast-feed.1

  • Importance of informing patients of other important precautionary information.1 (See Cautions.)


Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.

Please refer to the ASHP Drug Shortages Resource Center for information on shortages of one or more of these preparations.

* available from one or more manufacturer, distributor, and/or repackager by generic (nonproprietary) name



Dosage Forms


Brand Names




250 mg*

Chlorzoxazone Tablets

375 mg*

Chlorzoxazone Tablets


Vertical Pharmaceuticals

500 mg*

Chlorzoxazone Tablets

750 mg*

Chlorzoxazone Tablets


Vertical Pharmaceuticals

AHFS DI Essentials™. © Copyright 2021, Selected Revisions March 2, 2020. American Society of Health-System Pharmacists, Inc., 4500 East-West Highway, Suite 900, Bethesda, Maryland 20814.


1. Vertical Pharmaceuticals. Lorzone (chlorzoxazone) prescribing information. Bridgewater, NJ; 2017 Jan.

18. SCHWAB RS. MUSCLE RELAXANTS. Practitioner. 1964; 192:104-8.

24. CONNEY AH, BURNS JJ. Physiological disposition and metabolic fate of chlorzoxazone (paraflex) in man. J Pharmacol Exp Ther. 1960; 128:340-3.

100. Actavis. Chlorzoxazone tablets 500 mg prescribing information. Parsippany, NJ; 2015 Mar.

101. Powers BJ, Cattau EL Jr, Zimmerman HJ. Chlorzoxazone hepatotoxic reactions. Arch Intern Med. 1986; 146:1183-6.

102. Mikart. Chlorzoxazone tablets 250 mg prescribing information. Atlanta, GA; 2018 Aug.

103. See S, Ginzburg R. Skeletal muscle relaxants. Pharmacotherapy. 2008; 28:207-13.

104. van Tulder MW, Touray T, Furlan AD et al. Muscle relaxants for non-specific low back pain. Cochrane Database Syst Rev. 2003; :CD004252.

105. Roelofs PD, Deyo RA, Koes BW et al. Non-steroidal anti-inflammatory drugs for low back pain. Cochrane Database Syst Rev. 2008; :CD000396.

106. Chou R, Qaseem A, Snow V et al. Diagnosis and treatment of low back pain: a joint clinical practice guideline from the American College of Physicians and the American Pain Society. Ann Intern Med. 2007; 147:478-91.

107. Institute for Clinical Systems Improvement. Health care guideline: adult acute and subacute low back pain. 15th ed. Bloomington, MN; 2012 Jan. From the ICSI website

108. Toth PP, Urtis J. Commonly used muscle relaxant therapies for acute low back pain: a review of carisoprodol, cyclobenzaprine hydrochloride, and metaxalone. Clin Ther. 2004; 26:1355-67.

109. Qaseem A, Wilt TJ, McLean RM et al. Noninvasive Treatments for Acute, Subacute, and Chronic Low Back Pain: A Clinical Practice Guideline From the American College of Physicians. Ann Intern Med. 2017; 166:514-530.

110. Friedman BW, Cisewski D, Irizarry E et al. A Randomized, Double-Blind, Placebo-Controlled Trial of Naproxen With or Without Orphenadrine or Methocarbamol for Acute Low Back Pain. Ann Emerg Med. 2018; 71:348-356.e5.

111. Spence MM, Shin PJ, Lee EA et al. Risk of injury associated with skeletal muscle relaxant use in older adults. Ann Pharmacother. 2013 Jul-Aug; 47:993-8.

112. Friedman BW, Irizarry E, Solorzano C et al. A Randomized, Placebo-Controlled Trial of Ibuprofen Plus Metaxalone, Tizanidine, or Baclofen for Acute Low Back Pain. Ann Emerg Med. 2019;

113. Friedman BW, Dym AA, Davitt M et al. Naproxen With Cyclobenzaprine, Oxycodone/Acetaminophen, or Placebo for Treating Acute Low Back Pain: A Randomized Clinical Trial. JAMA. 2015; 314:1572-80.

b. AHFS drug information 2020. Snow EK, ed. Chlorzoxazone. Bethesda, MD: American Society of Health-System Pharmacists; 2020.