PARAFLEX
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PARAFLEX (PARAFLEX).
Centrally acting muscle relaxant; inhibits polysynaptic reflexes at the spinal cord level, possibly by depressing the central nervous system.
| Metabolism | Hepatic via hydrolysis to chlorzoxazone and subsequent glucuronidation and sulfation. |
| Excretion | Renal excretion of unchanged drug and metabolites accounts for approximately 50% of an oral dose; fecal excretion accounts for about 20%. |
| Half-life | Terminal elimination half-life is approximately 2–3 hours, allowing for multiple daily dosing. |
| Protein binding | Approximately 95% bound to plasma proteins, primarily albumin. |
| Volume of Distribution | Volume of distribution is approximately 1.5 L/kg, indicating extensive tissue distribution. |
| Bioavailability | Oral bioavailability is approximately 80% due to first-pass metabolism. |
| Onset of Action | Oral administration: onset of action occurs within 30 minutes. |
| Duration of Action | Duration of action is approximately 4–6 hours, necessitating dosing three to four times daily. |
250-500 mg orally once daily, may increase to 500 mg twice daily if needed. Maximum 500 mg/day.
| Dosage form | TABLET |
| Renal impairment | GFR 30-50 mL/min: 250 mg once daily. GFR 15-29 mL/min: 250 mg every other day. GFR <15 mL/min: not recommended. |
| Liver impairment | Child-Pugh A: no adjustment. Child-Pugh B: 250 mg once daily. Child-Pugh C: not recommended. |
| Pediatric use | Not recommended for pediatric use due to lack of safety and efficacy data. |
| Geriatric use | Start at 250 mg once daily; increase cautiously. Monitor for renal function and adverse effects. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for PARAFLEX (PARAFLEX).
| Breastfeeding | Chlorzoxazone is excreted into breast milk in small amounts. M/P ratio is not established. Use caution in nursing mothers; consider risk of infant sedation and potential adverse effects. Monitor infant for drowsiness or feeding difficulties. |
| Teratogenic Risk | Paraflex (chlorzoxazone) is classified as FDA pregnancy category C. Animal studies have shown adverse effects on fetal development, but no adequate human studies exist. First trimester: Potential risk of teratogenicity; use only if clearly needed. Second and third trimesters: No known specific risks, but avoid unnecessary use. Near term: Theoretical risk of neonatal muscle weakness or CNS depression. |
■ FDA Black Box Warning
Paraflex is not known to have a black box warning.
| Serious Effects |
["Hypersensitivity to chlorzoxazone or any component","Severe hepatic impairment"]
| Precautions | ["May cause drowsiness, dizziness, or impaired mental/physical abilities","Use caution when driving or operating machinery","Potential for hepatotoxicity with chronic high-dose use","May be habit-forming with prolonged use"] |
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| Fetal Monitoring | Monitor maternal liver function tests (hepatotoxicity risk). Monitor for symptoms of CNS depression in mother and fetus (e.g., sedation, dizziness). Assess fetal heart rate and growth if used chronically near term. |
| Fertility Effects | No data available on chlorzoxazone effects on human fertility. Animal studies have not shown significant reproductive impairment, but theoretical risk exists due to CNS effects. |