OSMOLEX ER
Clinical safety rating: caution
Comprehensive clinical and safety monograph for OSMOLEX ER (OSMOLEX ER).
Trihexyphenidyl is a centrally acting anticholinergic agent that blocks muscarinic receptors in the striatum, helping to restore the balance between acetylcholine and dopamine in the basal ganglia, thereby reducing extrapyramidal symptoms.
| Metabolism | Trihexyphenidyl is hydroxylated in the liver; metabolites are excreted renally. |
| Excretion | Primarily renal (60-80% as unchanged drug and glucuronide conjugates), biliary/fecal (20-40%) |
| Half-life | Terminal elimination half-life is 5-8 hours in healthy adults; prolonged in renal impairment (up to 16 hours in severe impairment). |
| Protein binding | 10-20% bound to plasma proteins, primarily albumin. |
| Volume of Distribution | 0.8-1.4 L/kg, indicating distribution into total body water. |
| Bioavailability | Oral: 70-85% (immediate-release); OSMOLEX ER: approximately 60-70% relative to immediate-release due to first-pass metabolism. |
| Onset of Action | Oral: 30-60 minutes (immediate-release formulation); OSMOLEX ER (extended-release): 1-2 hours. |
| Duration of Action | Oral (immediate-release): 4-6 hours; OSMOLEX ER (extended-release): 8-12 hours due to controlled release. |
Initial: 1 mg orally once daily; titrate by 1 mg every 3-5 days based on response and tolerability. Maximum: 8 mg once daily. Administer at bedtime.
| Dosage form | TABLET, EXTENDED RELEASE |
| Renal impairment | No dosage adjustment required for mild to moderate renal impairment. For severe renal impairment (CrCl <30 mL/min) or ESRD, use is not recommended due to lack of studies. |
| Liver impairment | Mild hepatic impairment (Child-Pugh A): No adjustment. Moderate to severe (Child-Pugh B or C): Not recommended. |
| Pediatric use | Safety and efficacy not established in pediatric patients (<18 years). |
| Geriatric use | Initial dose: 0.5 mg orally once daily. Titrate by 0.5 mg increments. Maximum: 4 mg once daily. Use with caution due to increased risk of adverse effects (e.g., hallucinations, confusion). |
| 1st trimester | Consult provider |
| 2nd trimester | Consult provider |
| 3rd trimester | Consult provider |
Clinical note
Comprehensive clinical and safety monograph for OSMOLEX ER (OSMOLEX ER).
| Breastfeeding | Excreted into breast milk in low amounts; M/P ratio not reported. Limited data; caution in nursing infants due to potential for CNS depression and withdrawal. Monitor infant for sedation, poor feeding, and respiratory depression. |
| Teratogenic Risk | Pregnancy Category C. First trimester: limited human data; animal studies show increased fetal resorptions and skeletal abnormalities at high doses. Second/third trimester: risk of neonatal withdrawal syndrome (tremors, hypertonia, poor feeding) with chronic use. Avoid in pregnancy unless benefit outweighs risk. |
■ FDA Black Box Warning
None
| Serious Effects |
["Hypersensitivity to trihexyphenidyl or any component","Narrow-angle glaucoma","Obstructive uropathy","Severe ulcerative colitis","Myasthenia gravis","Tardive dyskinesia (relative)"]
| Precautions | ["May impair mental and/or physical abilities; caution with driving or operating machinery","Use cautiously in patients with glaucoma, urinary retention, prostatic hypertrophy, or GI obstruction","May cause constipation, dry mouth, blurred vision, and cognitive impairment in elderly","Potential for abuse and dependence"] |
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| Fetal Monitoring |
| Monitor maternal blood pressure, heart rate, and signs of toxicity (dyskinesia, hallucinations). In third trimester, assess fetal movement and heart rate; neonatal monitoring for withdrawal symptoms for 48-72 hours post-delivery. |
| Fertility Effects | Decreased fertility noted in animal studies; clinical significance in humans unknown. May impair spermatogenesis or ovarian function at high doses. |