Can orphenadrine be given intramuscularly (IM)?

Can Orphenadrine Be Given Intramuscularly?

Yes, orphenadrine can be administered intramuscularly (IM), and this route is clinically established with proven bioavailability that equals or exceeds oral administration. 1

Route of Administration

• Orphenadrine is available in both oral and intravenous formulations, as confirmed by perioperative guidelines that describe it as "an oral and intravenous centrally acting skeletal muscle relaxant." 2

• The intramuscular route is a valid and effective option for orphenadrine administration. Research demonstrates that IM injection of orphenadrine hydrochloride achieves bioavailability equal to or greater than oral tablets. 1

• IM administration provides significantly higher plasma concentrations in the first hour compared to oral dosing, making it the preferred route when rapid drug effects are clinically required. 1

Clinical Context and Pharmacology

• Orphenadrine is structurally similar to diphenhydramine and functions as a centrally acting skeletal muscle relaxant with anticholinergic properties. 2

• The medication is commonly administered both preoperatively and perioperatively in clinical practice. 2

• Orphenadrine does not act directly on skeletal muscle; its precise mechanism of action remains unclear but appears to involve anticholinergic effects. 2

Important Safety Considerations

• Anticholinergic adverse effects include confusion, anxiety, tremors, urinary retention, dry mouth, constipation, and cardiovascular instability. 2

• Use with extreme caution in patients with tachycardia, cardiac decompensation, coronary insufficiency, cardiac arrhythmias, and in elderly patients. 2

• Absolute contraindications include gastrointestinal tract obstruction, stenosing peptic ulcer disease, prostatic hypertrophy or bladder outlet obstruction, glaucoma, and myasthenia gravis. 2

• Drug interactions: Orphenadrine potentiates anticholinergic effects when combined with other medications having anticholinergic properties. 2

• Pediatric toxicity warning: Orphenadrine can cause profound toxicity in children after ingestion of small doses, with central anticholinergic syndrome manifesting as hallucinations and severe agitation. 3

Dosing Information

• Standard IM dosing for orphenadrine hydrochloride is 40 mg, based on bioavailability studies comparing IM to oral routes. 1

• The oral equivalent dose is approximately 50 mg, though IM administration achieves higher early plasma concentrations. 1