Brand name: Lorzone
Drug class: Centrally Acting Skeletal Muscle Relaxants
VA class: MS200
CAS number: 95-25-0

Medically reviewed by Drugs.com on Feb 20, 2025. Written by ASHP.

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.^{1 18}

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

Administration

Oral Administration

Administer orally.^{1 100 102}

Dosage

Adults

Muscular Conditions

Oral

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

Contraindications

• Known intolerance to chlorzoxazone.^{1 100 102}

Warnings/Precautions

Warnings

Hepatotoxicity

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

Pregnancy

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

Lactation

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

Drug Interactions

Specific Drugs

Chlorzoxazone Pharmacokinetics

Absorption

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

Onset

Usually within 1 hour.^b

Duration

3–4 hours.^b

Distribution

Extent

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.^{1 24}

Elimination Route

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

Half-life

66 minutes.^b

Stability

Storage

Oral

Tablets

20–25°C.^{1 100 102}

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.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.)

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

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