Chlorzoxazone

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}

If pharmacologic therapy is required for acute low back pain (usually a benign and self-limiting condition^{105 106 108} ), 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}

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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

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

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}

Actions

• 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

Comparative Pricing

This pricing information is subject to change at the sole discretion of DS Pharmacy. This pricing information was updated 02/2013. Actual costs to patients will vary depending on the use of specific retail or mail-order locations and health insurance copays.

Chlorzoxazone 500MG Tablets (WATSON LABS): 90/$17.99 or 180/$24.98

Parafon Forte DSC 500MG Tablets (MCNEIL): 30/$92.99 or 90/$244.97