CHLORZOXAZONE
Clinical safety rating: caution
Comprehensive clinical and safety monograph for CHLORZOXAZONE (CHLORZOXAZONE).
Chlorzoxazone acts centrally on the spinal cord and subcortical areas of the brain to inhibit multisynaptic reflex arcs involved in producing and maintaining muscle spasm. It may also have some sedative effects.
| Metabolism | Hepatic, primarily via CYP2E1, also CYP1A2 and CYP3A4 |
| Excretion | Primarily hepatic metabolism followed by renal excretion of metabolites; <1% excreted unchanged in urine; minor biliary/fecal elimination. |
| Half-life | Terminal elimination half-life approximately 1–2 hours; clinically relevant for muscle relaxant effect duration. |
| Protein binding | Approximately 90–95% bound, primarily to albumin. |
| Volume of Distribution | 0.46–0.64 L/kg; indicates distribution into total body water. |
| Bioavailability | Oral: nearly complete; rapidly absorbed with extensive first-pass metabolism; systemic bioavailability approximately 30–50% due to first-pass effect. |
| Onset of Action | Oral: 30–60 minutes. |
| Duration of Action | 3–6 hours; effects may last up to 6 hours. |
250-500 mg orally 3-4 times daily, maximum 750 mg 4 times daily.
| Dosage form | TABLET |
| Renal impairment | No specific guidelines; use with caution in severe renal impairment (GFR <30 mL/min) due to potential accumulation of active metabolite. |
| Liver impairment | Contraindicated in hepatic impairment; avoid use in Child-Pugh class B or C due to risk of hepatotoxicity. |
| Pediatric use | Not established; safety and efficacy not studied in pediatric patients. |
| Geriatric use | Initiate at lower end of dosing range (250 mg 3-4 times daily); monitor for CNS effects (dizziness, drowsiness) and liver function. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for CHLORZOXAZONE (CHLORZOXAZONE).
| Breastfeeding | Not recommended during breastfeeding due to potential for sedation in the infant. No M/P ratio data available. |
| Teratogenic Risk | Teratogenic risk in humans is not well-studied. No major teratogenic effects have been reported in animal studies. However, as with all medications, use during pregnancy only if clearly needed and after weighing risks vs. benefits. Avoid during first trimester unless necessary. |
| Fetal Monitoring |
■ FDA Black Box Warning
None
| Serious Effects |
Hypersensitivity to chlorzoxazone or any component of the formulation; impaired hepatic function
| Precautions | May cause drowsiness, dizziness, or impaired coordination. Caution in patients with hepatic impairment. Discontinue if hypersensitivity reactions occur. Avoid concurrent use with alcohol or other CNS depressants. |
| Food/Dietary | No significant food interactions. Take with or without food. Grapefruit juice may increase drug levels; avoid large quantities. |
| Clinical Pearls |
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| No specific fetal monitoring required. Observe for maternal adverse effects such as hepatotoxicity or hypersensitivity reactions. No routine laboratory monitoring specified. |
| Fertility Effects | No known effect on human fertility from available data. |
| Chlorzoxazone is a centrally acting muscle relaxant used for acute musculoskeletal pain. Onset of action is within 1 hour; peak effect at 1-2 hours. Monitor for hepatotoxicity, especially with prolonged use or high doses. Can cause drowsiness and impair motor skills; avoid concurrent use with alcohol or other CNS depressants. Tablets may be crushed for patients with swallowing difficulties. |
| Patient Advice | Take exactly as prescribed; do not increase dose or frequency. · May cause drowsiness or dizziness; avoid driving or operating machinery until you know how it affects you. · Avoid alcohol and other CNS depressants while taking this medication. · Report signs of liver problems: dark urine, yellowing of eyes/skin, persistent nausea, abdominal pain. · Do not suddenly stop taking if used long-term; taper under medical supervision to avoid withdrawal. |