Overview
Chlorzoxazone is a centrally-acting skeletal muscle relaxant used as an adjunct to rest and physical therapy for acute painful musculoskeletal conditions. Its exact mechanism is not fully understood but appears related to CNS depression rather than direct muscle relaxation. The drug is generally well-tolerated but carries a rare risk of serious hepatotoxicity requiring monitoring.
Mechanism of Action
Chlorzoxazone acts centrally in the spinal cord and subcortical areas of the brain to inhibit polysynaptic reflexes involved in muscle spasm. It does not directly relax skeletal muscles at the neuromuscular junction but produces its effects through CNS depression, likely by interfering with reflex arcs involved in maintaining muscle tone.
Indications
- Adjunct therapy for acute, painful musculoskeletal conditions
- Relief of discomfort associated with muscle spasms
- Used with rest, physical therapy, and other supportive measures
Common Doses
- 250 mg
- 375 mg
- 500 mg
- 750 mg
Dosage
Adults: 375-750 mg three to four times daily. Start with lower doses (375-500 mg) and titrate upward based on response. Maximum daily dose typically 3000 mg. Reduce dosage as improvement occurs. Available in 250 mg, 375 mg, 500 mg, and 750 mg tablets.
Black Box Warning
Black Box Warning: Rare but potentially fatal hepatocellular toxicity reported
Contraindications
- Known hypersensitivity or intolerance to chlorzoxazone
- Active liver disease or history of drug-induced hepatotoxicity
- Concurrent use with other CNS depressants without careful monitoring
Side Effects
- Common: Drowsiness, dizziness, lightheadedness, malaise
- Gastrointestinal: Nausea, vomiting, diarrhea, possible gastrointestinal bleeding (rare)
- Dermatological: Allergic skin rashes, petechiae, ecchymoses
- Neurological: Over-stimulation, headache
- Urinary: Orange or red-purple urine discoloration (harmless phenolic metabolite)
- Serious: Hepatotoxicity, angioedema, anaphylaxis (rare)
Interactions
- Alcohol: Additive CNS depression and hepatotoxicity risk
- CNS depressants (benzodiazepines, opioids, barbiturates): Enhanced sedation and respiratory depression
- Hepatotoxic drugs: Increased risk of liver injury
- MAO inhibitors: Potential for enhanced effects
Counseling Points
- May cause drowsiness - avoid driving or operating machinery until effects known
- Report any signs of liver problems immediately: yellow skin/eyes, dark urine, abdominal pain, unusual fatigue
- Avoid alcohol and other sedating medications
- Urine may turn orange or red-purple - this is harmless
- Take with food if gastrointestinal upset occurs
- Do not exceed prescribed dosage
- Use as adjunct to rest and physical therapy, not as sole treatment