PARAFLEX
Clinical safety rating
cautionComprehensive 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. |
| Molecular Weight | 179.6 |
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 | Avoid. Chlorzoxazone (active ingredient) is a central muscle relaxant with limited human data; animal studies not available. Potential fetal risks cannot be ruled out. |
| 2nd trimester | Avoid. Same risks as first trimester; no established safety profile. |
| 3rd trimester | Avoid. Potential neonatal adverse effects including sedation and withdrawal; use only if clearly needed. |
Clinical note
Comprehensive clinical and safety monograph for PARAFLEX (PARAFLEX).
| Placental transfer | Unknown. Molecular weight (179.6 Da) suggests potential for placental crossing, but no studies available. |
| Breastfeeding | No data on excretion into breast milk. Given the lack of information, it is generally recommended to avoid breastfeeding while taking Paraflex due to potential sedation in the infant. |
| Lactation Rating | L4 (Probably Hazardous) - No data, but potential for infant sedation. |
| 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. |
| 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. |
■ FDA Black Box Warning
Paraflex is not known to have a black box warning.
| Serious Effects |
Hypersensitivity to chlorzoxazone or any componentSevere hepatic impairmentConcurrent use of alcohol or CNS depressants may potentiate effects; not absolute but caution advised
| 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 |
| Food/Dietary | Avoid alcohol consumption due to increased risk of hepatotoxicity and CNS depression. No specific food interactions; take with or without food. However, taking with food may reduce gastrointestinal upset. |
| Clinical Pearls | ParaFlex (chlorzoxazone) is a centrally acting muscle relaxant used for acute musculoskeletal pain. It should not be used for longer than 2-3 weeks due to lack of evidence for chronic use and potential hepatotoxicity. Monitor liver enzymes in patients with pre-existing hepatic impairment. Onset of action is within 1 hour; peak effect at 1-2 hours. Combine with rest and physical therapy for optimal outcome. |
| Patient Advice | Take this medication exactly as prescribed; do not exceed recommended dose or duration. · May cause drowsiness or dizziness; avoid driving or operating heavy machinery until you know how it affects you. · Avoid alcohol consumption as it may increase drowsiness and risk of liver damage. · Report symptoms of liver toxicity (e.g., yellowing of skin/eyes, dark urine, abdominal pain) immediately. · Do not discontinue abruptly; taper under medical advice if used for more than a few weeks. |
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