A fixed-dose combination (FDC) antipsychotic medication containing Risperidone, an atypical antipsychotic, and Trihexyphenidyl, an anticholinergic agent. This combination is primarily used in the management of schizophrenia and other psychotic disorders where extrapyramidal symptoms (EPS) induced by Risperidone are a significant concern. The rationale is to provide the therapeutic benefits of Risperidone while proactively mitigating its dopamine D2 receptor blockade-induced motor side effects. Its use as a first-line FDC is debated, and it is often considered after EPS emerge on Risperidone monotherapy.
Adult: One tablet (Risperidone 4mg + Trihexyphenidyl 2mg) once daily, usually at bedtime. Initiation at lower doses (e.g., Risperidone 2mg) is recommended, making this FDC more suitable for maintenance than initiation.
Note: Administer orally with or without food. Taking at bedtime can minimize daytime sedation and orthostatic hypotension. Tablet should be swallowed whole with a glass of water. Do not crush or chew. Avoid abrupt discontinuation.
Risperidone exerts its antipsychotic effect primarily through high-affinity antagonism of dopamine D2 and serotonin 5-HT2A receptors in the mesolimbic pathway. Its 5-HT2A antagonism is thought to contribute to a lower incidence of EPS compared to typical antipsychotics and efficacy against negative symptoms. Trihexyphenidyl is a centrally-acting antimuscarinic agent that blocks acetylcholine receptors in the striatum. This counteracts the excessive cholinergic activity that results from Risperidone's D2 blockade in the nigrostriatal pathway, thereby preventing or treating drug-induced parkinsonism, dystonia, and akathisia.
Pregnancy: Category C (US FDA). Risperidone: Limited human data, potential risk. Trihexyphenidyl: No well-controlled studies. Use only if potential benefit justifies potential fetal risk. Neonates exposed in 3rd trimester are at risk for EPS and withdrawal. Register with pregnancy registry.
Driving: May impair mental and/or physical abilities required for driving or operating machinery, especially during initial treatment and dose adjustments. Patients should be cautioned.
| Other CNS Depressants (Alcohol, Benzodiazepines, Opioids) | Additive sedation, respiratory depression, impaired motor skills. | Major |
| Other Anticholinergics (Tricyclic Antidepressants, 1st gen antihistamines, Oxybutynin) | Additive anticholinergic toxicity: delirium, hyperthermia, ileus, urinary retention. | Major |
| Levodopa/Carbidopa | Trihexyphenidyl may enhance effects; Risperidone may antagonize effects of Levodopa. | Moderate |
| CYP2D6 Inhibitors (Fluoxetine, Paroxetine, Quinidine) | Increase Risperidone plasma levels, increasing toxicity risk. May also affect Trihexyphenidyl. | Major |
| CYP2D6 Inducers (Carbamazepine, Rifampicin) | Decrease Risperidone plasma levels, reducing efficacy. | Major |
| Drugs prolonging QT interval (Class Ia/III antiarrhythmics, Macrolides, Fluoroquinolones) | Additive risk of torsades de pointes. | Major |
| Antihypertensives | Enhanced hypotensive effect. | Moderate |
| Metoclopramide | Increased risk of EPS. | Moderate |
Same composition (Risperidone (4mg) + Trihexyphenidyl (2mg)), different brands: