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Chlorzoxazone

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

Medically reviewed by Drugs.com on Feb 21, 2022. Written by ASHP.

Introduction

Centrally acting skeletal muscle relaxant.

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.

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

Various skeletal muscle relaxants appear to have comparable efficacy for low back pain relief.

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

Chlorzoxazone Dosage and Administration

Administration

Oral Administration

Administer orally.

Dosage

Adults

Muscular Conditions
Oral

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

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

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

Cautions for Chlorzoxazone

Contraindications

  • Known intolerance to chlorzoxazone.

Warnings/Precautions

Warnings

Hepatotoxicity

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

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

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.

CNS Effects

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

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

Sensitivity Reactions

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

Specific Populations

Pregnancy

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

Lactation

Not known whether chlorzoxazone is distributed into milk.

Geriatric Use

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

Common Adverse Effects

Drowsiness, dizziness.

Interactions for Chlorzoxazone

Specific Drugs

Drug

Interaction

Comments

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

Potential additive CNS depression

Use concomitantly with caution

Chlorzoxazone Pharmacokinetics

Absorption

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

Onset

Usually within 1 hour.

Duration

3–4 hours.

Distribution

Extent

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

Elimination

Metabolism

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

Elimination Route

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

Half-life

66 minutes.

Stability

Storage

Oral

Tablets

20–25°C.

Actions

  • CNS depressant with sedative and skeletal muscle relaxant effects.

  • 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.

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.

  • 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.

  • 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.

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

  • 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.

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

Chlorzoxazone

Routes

Dosage Forms

Strengths

Brand Names

Manufacturer

Oral

Tablets

250 mg*

Chlorzoxazone Tablets

375 mg*

Chlorzoxazone Tablets

Lorzone

Vertical Pharmaceuticals

500 mg*

Chlorzoxazone Tablets

750 mg*

Chlorzoxazone Tablets

Lorzone

Vertical Pharmaceuticals

AHFS DI Essentials™. © Copyright 2022, Selected Revisions March 2, 2020. American Society of Health-System Pharmacists, Inc., 4500 East-West Highway, Suite 900, Bethesda, Maryland 20814.

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