NORFLEX
Clinical safety rating: caution
Comprehensive clinical and safety monograph for NORFLEX (NORFLEX).
Orphenadrine is a centrally acting skeletal muscle relaxant with anticholinergic and local anesthetic properties. It acts primarily by blocking cholinergic receptors in the central nervous system, particularly in the reticular activating system, leading to reduced muscle spasm and rigidity.
| Metabolism | Primarily hepatic via cytochrome P450 enzymes, including CYP2D6 and CYP3A4; undergoes N-demethylation and hydroxylation. |
| Excretion | Renal: ~50% as unchanged drug and metabolites; biliary/fecal: ~40% as metabolites; <10% unchanged in feces. |
| Half-life | Terminal elimination half-life: 15-20 hours. Clinical context: Allows twice-daily dosing; steady-state reached in 3-5 days. |
| Protein binding | ~90% bound to plasma proteins, primarily albumin and alpha-1-acid glycoprotein. |
| Volume of Distribution | Vd: 2-3 L/kg (150-210 L for 70 kg adult). Indicates extensive tissue distribution. |
| Bioavailability | Oral: 85-90% (first-pass metabolism minimal). |
| Onset of Action | Oral: 30-60 minutes; IM: 15-30 minutes; IV: Immediate (within minutes). |
| Duration of Action | Oral: 4-6 hours; IM/IV: 2-4 hours. Clinical note: Skeletal muscle relaxation effects persist longer than sedation. |
Adults: 100 mg orally twice daily. Maximum dose: 200 mg/day.
| Dosage form | TABLET, EXTENDED RELEASE |
| Renal impairment | GFR 30-50 mL/min: 50 mg twice daily. GFR <30 mL/min: Not recommended. |
| Liver impairment | Child-Pugh A: 50 mg twice daily; Child-Pugh B: 50 mg once daily; Child-Pugh C: Contraindicated. |
| Pediatric use | Not recommended for patients under 12 years. Safety and efficacy not established. |
| Geriatric use | Initiate at 50 mg twice daily. Monitor for anticholinergic effects and cognitive impairment. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for NORFLEX (NORFLEX).
| Breastfeeding | Orphenadrine is excreted in breast milk in small amounts; M/P ratio not established. Reports of infant drowsiness. Caution advised, especially with higher doses or prolonged use. |
| Teratogenic Risk | Insufficient human data; animal studies show no fetal harm. First trimester: theoretical risk, avoid if possible. Second/third trimesters: no known malformations. Risk of uterine inertia and neonatal respiratory depression if used near term. |
| Fetal Monitoring |
■ FDA Black Box Warning
None
| Serious Effects |
["Hypersensitivity to orphenadrine or any component of the formulation","Glaucoma, particularly narrow-angle glaucoma","Gastrointestinal obstruction, paralytic ileus, or myasthenia gravis","Prostatic hypertrophy or bladder neck obstruction with urinary retention","Concurrent use with propoxyphene (risk of confusion, anxiety, tremors)"]
| Precautions | ["May impair mental alertness and physical coordination; caution in activities requiring alertness.","Anticholinergic effects: caution in patients with glaucoma, prostatic hypertrophy, or urinary retention.","Use with caution in elderly patients due to increased sensitivity to anticholinergic effects.","May precipitate tachycardia or transient syncope in patients with cardiac disorders.","Hepatic impairment: monitor liver function; dose adjustment may be needed."] |
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| Monitor maternal heart rate and blood pressure. Fetal heart rate monitoring if used near term. Assess neonatal respiratory status if administered during labor. |
| Fertility Effects | No specific human data on fertility effects. Animal studies do not indicate impaired fertility. Anticholinergic effects may theoretically affect reproductive function. |