Orphenadrine

Pronunciation

(or FEN a dreen)

U.S. Brand Names

Norflex™

Synonyms

Orphenadrine Citrate

Generic Available

Yes: Tablet

Canadian Brand Names

Norflex™; Orphenace®; Rhoxal-orphendrine

Use

Treatment of muscle spasm associated with acute painful musculoskeletal conditions; supportive therapy in tetanus

Pregnancy Risk Factor

Lactation

Excretion in breast milk unknown

Contraindications

Hypersensitivity to orphenadrine or any component of the formulation; glaucoma; GI obstruction; cardiospasm; myasthenia gravis

Warnings/Precautions

Use with caution in patients with CHF or cardiac arrhythmias; some products contain sulfites

Adverse Reactions

>10%:

Central nervous system: Drowsiness, dizziness

Ocular: Blurred vision

1% to 10%:

Cardiovascular: Flushing of face, tachycardia, syncope

Dermatologic: Rash

Gastrointestinal: Nausea, vomiting, constipation

Genitourinary: Decreased urination

Neuromuscular & skeletal: Weakness

Ocular: Nystagmus, increased intraocular pressure

Respiratory: Nasal congestion

<1%: Hallucinations, aplastic anemia

Overdosage/Toxicology

Symptoms of overdose include blurred vision, tachycardia, confusion, seizures, respiratory arrest, and dysrhythmias. There is no specific treatment for antihistamine overdose. Clinical toxicity is due to blockade of cholinergic receptors. Lethal dose is 2-3 g; treatment is generally symptomatic. For anticholinergic overdose with severe life-threatening symptoms, physostigmine 1-2 mg I.V. slowly, may be given to reverse these effects.

Drug Interactions

Substrate (minor) of CYP1A2, 2B6, 2D6, 3A4; Inhibits CYP1A2 (weak), 2A6 (weak), 2B6 (weak), 2C8/9 (weak), 2C19 (weak), 2D6 (weak), 2E1 (weak), 3A4 (weak)

Anticholinergic agents: May increase potential for anticholinergic adverse effects; includes drugs with high anticholinergic activity (diphenhydramine, TCAs, phenothiazines)

CNS depressants: Sedative effects may be additive. Monitor.

Levodopa: Effects may be decreased by orphenadrine. Monitor.

Ethanol/Nutrition/Herb Interactions

Ethanol: Avoid ethanol (may increase CNS depression).

Herb/Nutraceutical: St John's wort may decrease orphenadrine levels. Avoid valerian, St John's wort, kava kava, gotu kola (may increase CNS depression).

Mechanism of Action

Indirect skeletal muscle relaxant thought to work by central atropine-like effects; has some euphorigenic and analgesic properties

Pharmacodynamics/Kinetics

Onset of effect: Peak effect: Oral: 2-4 hours

Duration: 4-6 hours

Protein binding: 20%

Metabolism: Extensively hepatic

Half-life elimination: 14-16 hours

Excretion: Primarily urine (8% as unchanged drug)

Dosage

Adults:

Oral: 100 mg twice daily

I.M., I.V.: 60 mg every 12 hours

Administration

Do not crush sustained release drug product.

Patient Education

Take exactly as directed. Do not increase dose or discontinue without consulting prescriber. Do not chew or crush sustained release tablets. Do not use alcohol, prescriptive or OTC antidepressants, sedatives, or pain medications without consulting prescriber. You may experience drowsiness, dizziness, lightheadedness (avoid driving or engaging in tasks requiring alertness until response to drug is known); nausea or vomiting (small, frequent meals, frequent mouth care, or sucking hard candy may help); constipation (increased exercise, fluids, fruit, or fibers may help); or decreased urination (void before taking medication). Report excessive drowsiness or mental agitation, chest pain, skin rash, swelling of mouth/face, difficulty speaking, or vision changes. Pregnancy/breast-feeding precautions: Inform prescriber if you are or intend to become pregnant. Consult prescriber if breast-feeding.

Nursing Implications

Raise bed rails, institute safety measures, assist with ambulation

Dental Health: Effects on Dental Treatment

The peripheral anticholinergic effects of orphenadrine may decrease or inhibit salivary flow; normal salivation will return with cessation of drug therapy.

Dental Health: Vasoconstrictor/Local Anesthetic Precautions

No information available to require special precautions

Mental Health: Effects on Mental Status

Drowsiness and dizziness are common; may rarely cause hallucinations

Mental Health: Effects on Psychiatric Treatment

May rarely cause aplastic anemia; use caution with clozapine and carbamazepine; has been used to treat tardive dyskinesia and augment typical antipsychotics; clozapine is a better option; concurrent use with psychotropics may produce additive sedation

Dosage Forms

Injection, solution, as citrate: 30 mg/mL (2 mL) [contains sodium bisulfite]

Tablet, extended release, as citrate: 100 mg

References

Beech M, Hell C, and Nightingale P, "Central Anticholinergic Syndrome," Lancet , 1987, 1(8541):1089.

Boyson SJ, "Bethanechol for Anticholinergic Side Effects," Ann Neurol , 1988, 23(4):422-3.

Clarke B, Mair J, and Rudolf M, "Acute Poisoning With Orphenadrine," Lancet , 1985, 1(8442):1386.

Danze LK and Langdorf MI, "Reversal of Orphenadrine-Induced Ventricular Tachycardia With Physostigmine," J Emerg Med , 1991, 9(6):453-7.

International Brand Names

Biorphen® (GB); Derflex® (BR); Disipal® (BE, DK, GB, IL, IT, LU, NO, NZ, SE); Flexin® (IL); Lysantin® (DK); Norflex® (AU, BE); Norflex™ (CA); Norflex® (CR, DE, DO, FI, GT, HK, HN, LU, NZ, PA, SE, SV, TH, ZA); Orfenal® (CY, TH); Orphenace® (CA); Orphenadrine Hydrochloride® (GB); Orphipal® (IN); Phenerine® (ZA); Plenactol® (CL); Relaflex® (GT, HN, SV); Rhoxal-orphendrine (CA)

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