chlorzoxazone (Rx)

1-10%

Lightheadedness

Dizziness

Drowsiness

Excitement, Paradoxical

Somnolence

Malaise

Rash

Angioneurotic edema

<1%

Gastrointestinal hemorrhage (rare)

Hepatotoxicity (rare)

Anaphylaxis (rare)

Contraindications

Hypersensitivity to drug or excipients

Cautions

May cause CNS depression and impair physical and mental alertness; avoid performing tasks that require mental alertness

May take with food to avoid stomach upset

Poor tolerance by the elderly

Hepatotoxicity

• Serious (including fatal) hepatocellular toxicity has been reported rarely in patients receiving therapy; mechanism is unknown but appears to be idiosyncratic and unpredictable
• Factors predisposing patients to this rare event are not known
• Patients should be instructed to report early signs and/or symptoms of hepatotoxicity such as fever, rash, anorexia, nausea, vomiting, fatigue, right upper quadrant pain, dark urine, or jaundice; therapy should be discontinued immediately, and a physician consulted if signs or symptoms develop
• Therapy should also be discontinued if a patient develops abnormal liver enzymes (eg, AST, ALT, alkaline phosphatase and bilirubin)
• The concomitant use of alcohol or other central nervous system depressants may have an additive effect

Pregnancy Category: C

Lactation: not known if excreted in breast milk

Pregnancy Categories

A: Generally acceptable. Controlled studies in pregnant women show no evidence of fetal risk.

B: May be acceptable. Either animal studies show no risk but human studies not available or animal studies showed minor risks and human studies done and showed no risk.

C: Use with caution if benefits outweigh risks. Animal studies show risk and human studies not available or neither animal nor human studies done.

D: Use in LIFE-THREATENING emergencies when no safer drug available. Positive evidence of human fetal risk.

X: Do not use in pregnancy. Risks involved outweigh potential benefits. Safer alternatives exist.

NA: Information not available.

Mechanism of Action

Blocks interneuronal conduction in spinal cord and subcortical brain areas by depressing polysynaptic reflexes

Pharmacokinetics

Half-life: 66 min

Onset: 1 hr

Duration: 6-12 hr

Peak Plasma Time: 1-4 hr

Peak Plasm Concentration: 9-20 mcg/mL

Metabolism: Rapid hepatic glucuronidation

Metabolites: 6-hydroxychlorzoxazone (inactive; rapidly excreted in urine)

Excretion: Urine (primarily)