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. |
| Molecular Weight | 337.46 |
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 | There are no adequate and well-controlled studies in pregnant women. Animal studies have shown fetal abnormalities at high doses. Use only if potential benefit justifies potential risk to the fetus. |
| 2nd trimester | Limited human data; animal studies suggest risk. Monitor for fetal effects. Use if clearly needed. |
| 3rd trimester | May cause uterine contractions and premature labor. Avoid use near term unless necessary for maternal benefit. |
Clinical note
Comprehensive clinical and safety monograph for OSMOLEX ER (OSMOLEX ER).
| Placental transfer | Trihexyphenidyl crosses the placenta; specific degree not quantified. Molecular weight < 500 Da suggests transfer. |
| Breastfeeding | Trihexyphenidyl (active ingredient) is excreted into breast milk in small amounts. Monitor infant for anticholinergic effects (e.g., dry mouth, constipation, urinary retention). Consider benefits of breastfeeding and importance of drug to mother. |
■ FDA Black Box Warning
None
| Serious Effects |
Hypersensitivity to trihexyphenidyl or any componentNarrow-angle glaucomaPyloric or duodenal obstructionStenosing peptic ulcerProstatic hypertrophyBladder neck obstructionsMyasthenia gravisMegaesophagus
| 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 |
| Food/Dietary | No specific food interactions. However, alcohol may exacerbate CNS side effects; avoid concurrent use. |
Loading safety data…
| Lactation Rating | L3 - Moderately Safe |
| 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. |
| 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. |
| Clinical Pearls |
| OsmoLex ER (amantadine extended-release) is indicated for dyskinesia in Parkinson's disease patients on levodopa. Do not crush or chew the extended-release capsules; swallow whole. Use with caution in patients with renal impairment, as amantadine is renally excreted. Monitor for CNS effects (e.g., confusion, hallucinations, suicidal ideation). Avoid abrupt discontinuation to prevent neuroleptic malignant syndrome. |
| Patient Advice | Swallow the capsule whole; do not crush, chew, or cut it. · Take the medication at the same time each day, usually once daily in the morning. · Avoid alcohol while taking this medication as it may increase side effects. · Do not stop taking suddenly; consult your doctor for a tapering schedule. · Report any new or worsening mood changes, hallucinations, or thoughts of suicide. · If you have kidney problems, your dose may need adjustment; inform your doctor. |