spec sheet10 rows
Orphenadrine is an anticholinergic agent used as a skeletal muscle relaxant and, historically, in the management of Parkinsonism. A close structural analog of the antihistamine diphenhydramine, it combines first-generation antihistaminic and muscarinic-blocking activity with additional pharmacology that has been well characterized in vitro: binding and patch-clamp studies established it as an uncompetitive, voltage-dependent open-channel NMDA-receptor antagonist, and later work demonstrated concentration-dependent blockade of voltage-gated sodium channels implicated in its analgesic action. Despite this NMDA activity, its low channel affinity means it does not substitute for phencyclidine in drug-discrimination models, so overtly dissociative effects are minimal at therapeutic doses. In overdose the drug becomes a dangerous deliriant, with reported toxicity encompassing agitation, delirium, seizures, and, through sodium-channel effects, cardiac conduction disturbances.
- Relieves painful muscle spasm as a skeletal muscle relaxant
- Historically used for Parkinsonism and drug-induced parkinsonism
- Combination products pair it with analgesics for musculoskeletal pain
- Multi-target NMDA, muscarinic and sodium-channel blocker
- Dry mouth, blurred vision and urinary retention
- Sedation and dizziness
- Confusion, especially in older adults
- Delirium, seizures and heart-rhythm disturbances in overdose
Mechanism
Orphenadrine antagonizes receptors, which underlies both its anti-Parkinsonian use and its anticholinergic side-effect profile. It is a structural relative of diphenhydramine and shares H1 antihistamine activity that adds sedation. Distinctively, it is also an at receptors (a channel involved in pain signaling and, at higher blockade, dissociation), which contributes to its muscle-relaxant and analgesic effects and to its danger in overdose. It also has some sodium-channel-blocking activity. In overdose the combined muscarinic blockade drives an anticholinergic delirium, while the NMDA and sodium-channel effects add to cardiac and toxicity, making orphenadrine overdose particularly dangerous.
receptor fingerprint
receptorsAntagonist
receptorAntagonist
H1 receptorAntagonist
Safetyrisks and cautions, not medical advice
Orphenadrine has a narrow margin of safety and is notorious for dangerous, sometimes fatal overdoses; high doses cause anticholinergic delirium with realistic hallucinations, confusion and memory loss plus the full toxidrome (blurred vision, overheating, flushed dry skin, dry mouth, racing heart, urinary retention), and its sodium-channel and NMDA effects add seizures and heart-rhythm collapse. Combining it with other anticholinergics or CNS depressants stacks the anticholinergic burden and the risk. High anticholinergic burden, especially in older people and with long-term use, is linked to greater dementia risk. Not medical advice.
Subjective profileweighing the evidence above
The narrow margin of safety is the headline; overdoses are notoriously dangerous and sometimes fatal, bringing delirium, seizures and heart-rhythm collapse. It has a legitimate prescription role in painful muscle spasm, but gentler muscle relaxants exist and there is no case for experimenting with this one.
Resources
This entry is here for reference.
Research
- 1980first citedCentrally acting oral skeletal muscle relaxants
- 2009most recentInvolvement of voltage-gated sodium channels blockade in the analgesic effects of orphenadrine
- 1.Involvement of voltage-gated sodium channels blockade in the analgesic effects of orphenadrine
- 2.Death caused by orphenadine poisoning
- 3.Centrally acting oral skeletal muscle relaxants
- 4.Orphenadrine is an uncompetitive N-methyl-D-aspartate (NMDA) receptor antagonist: binding and patch clamp studies.
- 5.Evaluation of the phencyclidine-like discriminative stimulus effects of novel NMDA channel blockers in rats.
- 6.[Acute poisoning by orphenadrine].
6 listed here; entry last updated August 2026
Reviews
My notesprivate to this device
FAQ
How is orphenadrine different from a plain antihistamine?
It is a diphenhydramine analog but adds meaningful NMDA-receptor and sodium-channel blockade, which is why it works as a muscle relaxant and why overdoses are especially dangerous.
Is orphenadrine overdose dangerous?
Very. It is well known for a narrow safety margin, with overdoses causing delirium, seizures and life-threatening cardiac effects. There is no recreational upside to justify that risk.
Does it get you high?
No. High doses produce an unpleasant anticholinergic delirium, not euphoria, on top of serious physical toxicity.
Adverse effects
- Dry mouth, blurred vision and urinary retention
- Sedation and dizziness
- Confusion, especially in older adults
- Delirium, seizures and heart-rhythm disturbances in overdose