Orphenadrine is a centrally acting muscle relaxant that has been in clinical use for more than 50 years and has not been linked to liver injury or clinically apparent drug induced liver disease.
Orphenadrine (or fen' a dreen) is a centrally acting, nonopiate analgesic and muscle relaxant. It is a methyl derivative of diphenhydramine (a commonly used antihistamine), but its mechanism of action in causing analgesia and skeletal muscle relaxation is not well defined. Orphenadrine has anticholinergic activity and may act centrally on pain perception. Orphenadrine is currently used for the treatment of acute, painful musculoskeletal conditions and can be given orally or parenterally. Orphenadrine was approved for use as a muscle relaxant in the United States in 1957 and it is still in wide use. Orphenadrine is available in multiple generic forms as standard and extended release tablets of 100 mg. It is also available under commercial names such as Norgesic, Norflex, Deenar, Banflex, Disipal and X-Otag. The recommended dosage is 100 mg twice daily. Orphenadrine is also available in parenteral formulations under the names of Flexoject and Myolin. The parenteral dose recommendation is 60 mg either intravenously or intramuscularly twice daily. The most common side effects are those typical of anticholinergics including drowsiness, dry mouth, diaphoresis, flushing, confusion and visual disturbances. Orphenadrine also has a potential for abuse and fatal overdoses have been reported.
Despite its long clinical use, there is no evidence of hepatotoxicity with orphenadrine. Several cases of severe orphenadrine overdose with cardio-respiratory arrest and ischemic hepatic injury have been reported. Conventional doses of orphenadrine appear to be free of hepatic injury.
Likelihood score: E (Unlikely cause of clinically apparent liver injury).
REPRESENTATIVE TRADE NAMES
Orphenadrine – Generic, Norflex®
Autonomic Agents: Muscle Relaxants, Central
Product labeling at DailyMed, National Library of Medicine, NIH
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References updated: 30 January 2017
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