PARAFON FORTE DSC
Clinical safety rating: caution
Comprehensive clinical and safety monograph for PARAFON FORTE DSC (PARAFON FORTE DSC).
Chlorzoxazone acts on the central nervous system (CNS) at the spinal cord level, possibly by depressing polysynaptic reflexes, producing skeletal muscle relaxation without affecting neuromuscular transmission.
| Metabolism | Hepatic, primarily via glucuronidation and sulfation; minor cytochrome P450 involvement (CYP2E1, CYP1A2) |
| Excretion | Primarily renal (85-95% as glucuronide conjugates and unchanged drug; <5% fecal). |
| Half-life | 1-3 hours (terminal); clinically relevant for dosing intervals of 4-6 hours. |
| Protein binding | 10-25% (albumin). |
| Volume of Distribution | 0.8-1.0 L/kg; indicates distribution into total body water. |
| Bioavailability | Oral: 85-90% (first-pass metabolism ≤10%). |
| Onset of Action | Oral: 30-60 minutes (analgesic effect). |
| Duration of Action | 4-6 hours (analgesic); may be shorter in hepatic impairment. |
Adults: 4 g (500 mg x 8 tablets) orally every 6-8 hours as needed; maximum 8 g (16 tablets) per 24 hours.
| Dosage form | TABLET |
| Renal impairment | For acetaminophen component: No adjustment required for GFR >30 mL/min; reduce dosing interval to every 8 hours for GFR 10-30 mL/min; and every 12 hours for GFR <10 mL/min. Chlorzoxazone: consider cautious use; no specific guidelines. |
| Liver impairment | Acetaminophen: contraindicated in severe hepatic impairment (Child-Pugh C); reduce dose in moderate impairment (Child-Pugh B) to adult dose every 12 hours. Chlorzoxazone: use with caution in mild-to-moderate impairment; avoid in severe impairment. |
| Pediatric use | Not recommended for children under 12 years of age due to chlorzoxazone safety concerns. For acetaminophen: weight-based dosing 10-15 mg/kg/dose every 4-6 hours; maximum 75 mg/kg/day. |
| Geriatric use | Start at lower end of dosing range; monitor renal and hepatic function; maximum daily acetaminophen dose 3 g. Chlorzoxazone: use with caution due to increased risk of CNS effects. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for PARAFON FORTE DSC (PARAFON FORTE DSC).
| Breastfeeding | It is not known whether chlorzoxazone is excreted in human milk. The M/P ratio has not been established. Due to potential for adverse effects in the nursing infant (e.g., sedation), use caution; consider alternative agents with more safety data during breastfeeding. |
| Teratogenic Risk | Chlorzoxazone (Parafon Forte DSC) is a centrally acting muscle relaxant. Data on teratogenicity in humans are limited. In animal studies, no consistent teratogenic effects were observed at clinically relevant doses. First trimester: theoretical risk, but no confirmed human data; avoid unless essential. Second and third trimesters: no known specific fetal risks, but use only if clearly needed due to lack of robust safety data. |
■ FDA Black Box Warning
None
| Serious Effects |
Hypersensitivity to chlorzoxazone or any component; severe hepatic impairment; porphyria.
| Precautions | Hepatotoxicity (rare but severe, including fatal hepatic necrosis); discontinue if signs of liver injury; caution in hepatic impairment; may cause drowsiness or dizziness; avoid alcohol. |
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| Fetal Monitoring | Monitor maternal liver function tests (AST, ALT) periodically due to rare hepatotoxicity with chlorzoxazone. No specific fetal monitoring required; however, assess fetal growth and well-being if used chronically during pregnancy. |
| Fertility Effects | No documented effects on human fertility. Animal studies have not shown impaired fertility at therapeutic doses. |