ORPHENADRINE CITRATE
Clinical safety rating: safe
Other anticholinergic drugs can have additive effects Can cause drowsiness and dry mouth.
Orphenadrine citrate is a centrally acting muscle relaxant with anticholinergic properties. Its exact mechanism of action is not fully understood, but it is believed to exert its effects by blocking muscarinic acetylcholine receptors and possibly by acting as an NMDA receptor antagonist. It may also have local anesthetic and antihistaminic properties.
| Metabolism | Hepatic metabolism via cytochrome P450 enzymes (primarily CYP2D6 and CYP3A4) and other pathways; undergoes extensive first-pass metabolism. |
| Excretion | Primarily renal excretion of metabolites; less than 10% excreted unchanged. Also undergoes biliary excretion with fecal elimination of conjugates. |
| Half-life | Terminal elimination half-life is approximately 14 hours (range 11–20 hours) in adults; may be prolonged in elderly or hepatic impairment. |
| Protein binding | Approximately 20–30% bound to plasma proteins (mainly albumin). |
| Volume of Distribution | Approximately 1.0–1.6 L/kg, indicating extensive tissue distribution with highest concentrations in liver, lung, and kidney. |
| Bioavailability | Oral: approximately 50–70% due to first-pass metabolism; intramuscular: nearly 100%. |
| Onset of Action | Intravenous: 5–10 minutes; Intramuscular: 20–30 minutes; Oral: 30–60 minutes. |
| Duration of Action | Oral: 4–6 hours; Intravenous: 2–4 hours; sustained release formulations extend effect up to 12 hours. Onset and offset may be slower in elderly. |
100 mg orally twice daily. Maximum: 250 mg/day.
| Dosage form | INJECTABLE |
| Renal impairment | GFR 30-50 mL/min: decrease dose by 50%; GFR <30 mL/min: avoid use. |
| Liver impairment | Child-Pugh A: no adjustment; Child-Pugh B: reduce dose by 50%; Child-Pugh C: avoid use. |
| Pediatric use | Not recommended for use in children <12 years. For older children: 0.5-1 mg/kg/dose orally every 12 hours, max 100 mg/day. |
| Geriatric use | Start with 50 mg orally twice daily; increase cautiously due to anticholinergic sensitivity. Avoid in patients >80 years. |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Other anticholinergic drugs can have additive effects Can cause drowsiness and dry mouth.
| FDA category | Animal |
| Breastfeeding | Orphenadrine is excreted into breast milk in small amounts (M/P ratio not reported in literature). The relative infant dose is estimated to be low. However, anticholinergic effects (e.g., dry mouth, constipation, urinary retention) could potentially occur in the breastfed infant, especially with high maternal doses. Caution is advised. Monitor infant for signs of anticholinergic toxicity. |
| Teratogenic Risk |
■ FDA Black Box Warning
None.
| Common Effects | Dry mouth |
| Serious Effects |
Hypersensitivity to orphenadrine or any component of the formulation. Glaucoma (angle-closure). Prostatic hypertrophy or other urinary retention disorders. Pyloric or duodenal obstruction, stenosing peptic ulcers, cardiospasm (megaeosophagus). Myasthenia gravis. Concurrent use with monoamine oxidase inhibitors (MAOIs) within the past 14 days.
| Precautions | May cause anticholinergic effects such as dry mouth, blurred vision, urinary retention, and constipation. Use with caution in patients with glaucoma, prostatic hypertrophy, or gastrointestinal obstruction. May impair mental and physical abilities; caution when driving or operating machinery. Risk of abuse and dependence due to euphoric effects. Avoid abrupt discontinuation to prevent withdrawal symptoms. Elderly patients may be more sensitive to anticholinergic effects. |
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| Orphenadrine citrate is classified as FDA Pregnancy Category C. No adequate well-controlled studies in pregnant women. In animal studies, orphenadrine has been shown to have embryocidal effects in rats and rabbits at doses 3-4 times the human dose. First trimester: potential risk cannot be ruled out; use only if clearly needed. Second and third trimesters: limited data; avoid use near term due to possible anticholinergic effects on the fetus (e.g., tachycardia, paralytic ileus). There is no evidence of structural teratogenicity from human data. |
| Fetal Monitoring | No specific mandatory monitoring is required. However, during pregnancy, assess maternal heart rate and blood pressure due to potential anticholinergic effects (tachycardia, hypertension). Fetal heart rate monitoring may be considered if maternal anticholinergic symptoms occur. Monitor for preterm labor as anticholinergics may theoretically affect uterine tone. |
| Fertility Effects | Data on human fertility are lacking. Animal studies have not shown impaired fertility at clinically relevant doses. Anticholinergic effects do not directly impair reproductive function, but caution is warranted in women attempting conception due to unknown risks. |