Chlorzoxazone

Pronunciation

(klor ZOKS a zone)

U.S. Brand Names

Parafon Forte® DSC

Generic Available

Yes

Canadian Brand Names

Parafon Forte®; Strifon Forte®

Use

Symptomatic treatment of muscle spasm and pain associated with acute musculoskeletal conditions

Use - Dental

Treatment of muscle spasm and pain associated with acute temporomandibular joint pain

Pregnancy Risk Factor

Lactation

Excretion in breast milk unknown/not recommended

Contraindications

Hypersensitivity to chlorzoxazone or any component of the formulation; impaired liver function

Adverse Reactions

Frequency not defined.

Central nervous system: Dizziness, drowsiness lightheadedness, paradoxical stimulation, malaise

Dermatologic: Rash, petechiae, ecchymoses (rare), angioneurotic edema

Gastrointestinal: Nausea, vomiting, stomach cramps

Genitourinary: Urine discoloration

Hepatic: Liver dysfunction

Miscellaneous: Anaphylaxis (very rare)

Overdosage/Toxicology

Symptoms of overdose include nausea, vomiting, diarrhea, drowsiness, dizziness, headache, absent tendon reflexes, and hypotension. Treatment is supportive following attempts to enhance drug elimination. Dialysis, hemoperfusion, and osmotic diuresis have all been useful in reducing serum drug concentrations. The patient should be observed for possible relapses due to incomplete gastric emptying.

Drug Interactions

Substrate of CYP1A2 (minor), 2A6 (minor), 2D6 (minor), 2E1 (major), 3A4 (minor); Inhibits CYP2E1 (weak), 3A4 (weak)

CNS depressants: Effects may be increased by chlorzoxazone.

CYP2E1 inhibitors: May increase the levels/effects of chlorzoxazone. Example inhibitors include disulfiram, isoniazid, and miconazole.

Disulfiram: May increase chlorzoxazone concentration; monitor.

Isoniazid: May increase chlorzoxazone concentration; monitor.

Ethanol/Nutrition/Herb Interactions

Ethanol: Avoid ethanol (may increase CNS depression).

Mechanism of Action

Acts on the spinal cord and subcortical levels by depressing polysynaptic reflexes

Pharmacodynamics/Kinetics

Onset of action: ~1 hour

Duration: 6-12 hours

Absorption: Readily absorbed

Metabolism: Extensively hepatic via glucuronidation

Excretion: Urine (as conjugates)

Dosage

Oral:

Children: 20 mg/kg/day or 600 mg/m² /day in 3-4 divided doses

Adults: 250-500 mg 3-4 times/day up to 750 mg 3-4 times/day

Monitoring Parameters

Periodic liver functions tests

Patient Education

Take exactly as directed with food. Do not increase dose or discontinue without consulting prescriber. Do not use alcohol, prescriptive or OTC antidepressants, sedatives, or pain medications without consulting prescriber. May turn urine orange or red (normal). You may experience drowsiness, dizziness, lightheadedness (avoid driving or engaging in tasks that require alertness until response to drug is known); nausea, vomiting, or cramping (small, frequent meals, frequent mouth care, or sucking hard candy may help); postural hypotension (change position slowly when rising from sitting or lying or when climbing stairs); or constipation (increased exercise, fluids, fruit, or fiber may help). Report excessive drowsiness or mental agitation; palpitations, rapid heartbeat, or chest pain; skin rash or swelling of mouth or face; persistent diarrhea or constipation; or unusual weakness or bleeding. Pregnancy/breast-feeding precautions: Inform prescriber if you are or intend to become pregnant. Breast-feeding is not recommended.

Dental Health: Effects on Dental Treatment

No significant effects or complications reported

Dental Health: Vasoconstrictor/Local Anesthetic Precautions

No information available to require special precautions

Mental Health: Effects on Mental Status

Drowsiness is common; may produce depression or paradoxical stimulation

Mental Health: Effects on Psychiatric Treatment

May produce aplastic anemia and leukopenia; use caution with clozapine and carbamazepine

Dosage Forms

Caplet (Parafon Forte® DSC): 500 mg

Tablet: 250 mg, 500 mg

International Brand Names

Chlorzox® (TH); Clorzoxazon-Armedica® (RO); Clorzoxazona® (RO); Klorzoxazon Dak® (DK); Myoflex® (HR); Myoflexin® (HU); Paraflex® (DK, SE, TR, ZA); Parafon Forte® (CA); Solaxin® (HK, ID); Strifon Forte® (CA)

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