Class: Centrally Acting Skeletal Muscle Relaxants
VA Class: MS200
CAS Number: 95-25-0
Brands: Parafon Forte DSC
Introduction
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.a i
For low back pain, generally reserve skeletal muscle relaxants for adjunctive treatment when pain is unresponsive to OTC analgesics (e.g., NSAIAs).c d e f g h
Skeletal muscle relaxants less well tolerated than NSAIAs, and clinical superiority to NSAIAs not established for low back pain.c d e f g h
Various skeletal muscle relaxants appear to have comparable efficacy for low back pain reliefi c and are more effective than placebo.c e g
Initially, symptomatic control of acute low back pain focuses on providing sufficient comfort to allow maximum possible activity while awaiting spontaneous recovery; later, as aid to overcome specific activity intolerance.g
Because of rapid spontaneous recovery rate, efficacy of various therapies may be difficult to establish;g improvement of low back pain usually occurs within 2 weeks, substantial improvement within 4 weeks.f
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
Administration
Oral Administration
Administer orally.a i
Dosage
Adults
Muscular Conditions
Oral
Initially, 500 mg 3–4 times daily; if response inadequate, increase dosage to 750 mg 3–4 times daily.a i
When desired response is obtained, reduce dosage to lowest effective level.a i
Usual adult dosage: 250 mg 3–4 times daily.i
Cautions for Chlorzoxazone
Contraindications
Known intolerance to chlorzoxazone or any ingredient in the formulation.a i
Warnings/Precautions
Warnings
Hepatotoxicity
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
CNS Effects
Performance of activities requiring mental alertness or physical coordination may be impaired.a i
Concurrent use of other CNS depressants may potentiate CNS depression.a i (See Specific Drugs under Interactions.)
Sensitivity Reactions
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
Specific Populations
Pregnancy
Category C.i
Lactation
Not known whether chlorzoxazone is distributed into milk; use with caution.b
Pediatric Use
Safety and efficacy not established.a i
Common Adverse Effects
Drowsiness, dizziness.b
Interactions for Chlorzoxazone
Specific Drugs
Drug | Interaction | Comments |
|---|---|---|
CNS depressants (e.g., alcohol, barbiturates) | Potential additive CNS depressiona i | Use concomitantly with cautiona i |
Chlorzoxazone Pharmacokinetics
Absorption
Well absorbed following oral administration,b with peak plasma concentrations usually attained within 1–2 hours.a i
Onset
Usually within 1 hour.b
Duration
3–4 hours.b
Distribution
Extent
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
Elimination
Metabolism
Rapidly metabolized in the liver to 6-hydroxychlorzoxazone, an inactive metabolite.b
Elimination Route
Excreted principally in urine as a conjugated glucuronide; <1 % excreted as unchanged drug.a i
Half-life
66 minutes.b
Stability
Storage
Oral
Tablets
15–30°C.a i
ActionsActions
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
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.a i
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.a i
Importance of women informing clinicians if they are or plan to become pregnant or plan to breast-feed.a i
Importance of informing patients of other important precautionary information. (See Cautions.)
Preparations
Excipients in commercially available drug preparations may have clinically important effects in some individuals; consult specific product labeling for details.
* available from one or more manufacturer, distributor, and/or repackager by generic (nonproprietary) name
Routes | Dosage Forms | Strengths | Brand Names | Manufacturer |
|---|---|---|---|---|
Oral | Tablets | 250 mg* | Chlorzoxazone Tablets | Actavis |
500 mg* | Chlorzoxazone Tablets | Actavis, Teva, UDL | ||
Parafon Forte DSC Caplets (scored) | Ortho-McNeil |
Comparative Pricing
This pricing information is subject to change at the sole discretion of DS Pharmacy. This pricing information was updated 03/2011. Actual costs to patients will vary depending on the use of specific retail or mail-order locations and health insurance copays.
Chlorzoxazone 500MG Tablets (TEVA PHARMACEUTICALS USA): 90/$16.99 or 180/$20.98
Parafon Forte DSC 500MG Tablets (MCNEIL): 30/$92.99 or 90/$244.97
Disclaimer
This report on medications is for your information only, and is not considered individual patient advice. Because of the changing nature of drug information, please consult your physician or pharmacist about specific clinical use.
The American Society of Health-System Pharmacists, Inc. and Drugs.com represent that the information provided hereunder was formulated with a reasonable standard of care, and in conformity with professional standards in the field. The American Society of Health-System Pharmacists, Inc. and Drugs.com make no representations or warranties, express or implied, including, but not limited to, any implied warranty of merchantability and/or fitness for a particular purpose, with respect to such information and specifically disclaims all such warranties. Users are advised that decisions regarding drug therapy are complex medical decisions requiring the independent, informed decision of an appropriate health care professional, and the information is provided for informational purposes only. The entire monograph for a drug should be reviewed for a thorough understanding of the drug's actions, uses and side effects. The American Society of Health-System Pharmacists, Inc. and Drugs.com do not endorse or recommend the use of any drug. The information is not a substitute for medical care.
AHFS Drug Information. © Copyright, 1959-2011, Selected Revisions May 2007. American Society of Health-System Pharmacists, Inc., 7272 Wisconsin Avenue, Bethesda, Maryland 20814.
References
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. [IDIS 217157] [PubMed 3521519]
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.
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.].
c. van Tulder MW, Touray T, Furlan AD et al. Muscle relaxants for non-specific low back pain. The Cochrane Library. From their web site (). Accessed 11/10/2003.
d. van Tulder MW, Scholten RJPM, Kowes BW, et al. Non-steroidal anti-inflammatory drugs for low back pain. The Cochrane Library. From their web site (). Accessed 11/10/2003.
e. Department of Veterans Affairs Veterans Health Administration Office of Quality & Performance. Management of Person with Low Back Pain/Sciatica in Primary Care. From the web site (). Accessed 8/25/2003.
f. Agency for Healthcare Research and Quality. Adult low back pain. From the National Guideline Clearinghouse website. (). Accessed 8/25/2003.
g. U.S. Department of Health and Human Services. Public Health Service. Agency for Halath Care Policy and Research. Acute low back problems in adults. Clinical Practice Guideline.1994; No. 14
h. U.S. Department of Health and Human Services. Public Health Service. Agency for Health Care Policy and Research. Acute low back problems in adults: Assessment and treatment. Clinical Practice Guideline. Quick Reference Guide for Clinicians.1994; No. 14
i. Amide Pharmaceutical, Inc. Chlorzoxazone tablets prescribing information. Little Falls, NJ: 2002 Apr.
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