Class: Centrally Acting Skeletal Muscle Relaxants
VA Class: MS200
CAS Number: 95-25-0
Brands: Parafon Forte DSC
Medically reviewed on Nov 19, 2018
Centrally acting skeletal muscle relaxant.b
Uses for Chlorzoxazone
If pharmacologic therapy is required for acute low back pain (usually a benign and self-limiting condition105 106 108 ), an analgesic (e.g., acetaminophen, NSAIA) generally is recommended.104 105 106 108 117 Skeletal muscle relaxants may be used alone or in combination with analgesics for short-term relief; however, consider high incidence of adverse effects (e.g., CNS effects).104 106 107 108 Use skeletal muscle relaxants with caution and weigh risks against benefits.104 106 107 108
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
Usual adult dosage: 250 mg 3–4 times daily.i
Cautions for Chlorzoxazone
Serious, possibly fatal, hepatotoxicity reported rarely; may be associated with increased serum AST, ALT, alkaline phosphatase, and bilirubin concentrations and increased sulfobromophthalein sodium (BSP) retention.100 101 102 Mechanism unknown but appears idiosyncratic and unpredictable.100 101 102
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.100 102 a
Sensitivity reactions (e.g., urticaria, redness, petechiae, pruritus) reported rarely; angioedema and anaphylactic reactions are extremely rare.a i Use with caution in patients with allergies or with a history of allergic drug reactions.a b i
Not known whether chlorzoxazone is distributed into milk; use with caution.b
Common Adverse Effects
Interactions for Chlorzoxazone
CNS depressants (e.g., alcohol, barbiturates)
Usually within 1 hour.b
In dogs, found in plasma and fat; also present in liver, muscle, brain, and kidneys.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.b
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.a i
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.100 102
Importance of informing patients of other important precautionary information. (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
Parafon Forte DSC Caplets (scored)
AHFS DI Essentials. © Copyright 2019, Selected Revisions November 19, 2012. American Society of Health-System Pharmacists, Inc., 4500 East-West Highway, Suite 900, Bethesda, Maryland 20814.
100. McNeil Pharmaceutical. Parafon Forte DSC (chlorzoxazone) prescribing information (dated 1994 Dec 8). In: Physicians’ desk reference. 48th ed. Montvale, NJ: Medical Economics Company Inc; 1996:1581.
101. Powers BJ, Cattau EL Jr, Zimmerman HJ. Chlorzoxazone hepatotoxic reactions. Arch Intern Med. 1986; 146:1183-6. http://www.ncbi.nlm.nih.gov/pubmed/3521519?dopt=AbstractPlus
102. McNeil Pharmaceutical. Paraflex (chlorzoxazone) prescribing information (dated 1994 Dec 8). In: Physicians’ desk reference. 50th ed. Montvale, NJ: Medical Economics Company Inc; 1996:1580-1.
103. See S, Ginzburg R. Skeletal muscle relaxants. Pharmacotherapy. 2008; 28:207-13. http://www.ncbi.nlm.nih.gov/pubmed/18225966?dopt=AbstractPlus
104. van Tulder MW, Touray T, Furlan AD et al. Muscle relaxants for non-specific low back pain. Cochrane Database Syst Rev. 2003; :CD004252. http://www.ncbi.nlm.nih.gov/pubmed/12804507?dopt=AbstractPlus
105. Roelofs PD, Deyo RA, Koes BW et al. Non-steroidal anti-inflammatory drugs for low back pain. Cochrane Database Syst Rev. 2008; :CD000396. http://www.ncbi.nlm.nih.gov/pubmed/18253976?dopt=AbstractPlus
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. http://www.ncbi.nlm.nih.gov/pubmed/17909209?dopt=AbstractPlus
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 http://www.icsi.org/low_back_pain/adult_low_back_pain__8.html
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. http://www.ncbi.nlm.nih.gov/pubmed/15530999?dopt=AbstractPlus
117. Boothby LA, Doering PL, Hatton RC. Carisoprodol: a marginally effective skeletal muscle relaxant with serious abuse potential. Hosp Pharm. 2003; 38:337-45.
a. Ortho-McNeil. Parafon Forte DSC caplets prescribing information. Raritan, NJ; 2000 Aug.
b. AHFS drug information 2006. McEvoy GK, ed. Chloroxazone. Bethesda, MD: American Society of Health-System Pharmacists; 2006:[1385-6.].
i. Amide Pharmaceutical, Inc. Chlorzoxazone tablets prescribing information. Little Falls, NJ: 2002 Apr.
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