A fixed-dose combination (FDC) of an atypical antipsychotic (Risperidone) and an anticholinergic agent (Trihexyphenidyl). Risperidone is a benzisoxazole derivative with potent serotonin 5-HT2A and dopamine D2 receptor antagonism. Trihexyphenidyl is a synthetic antimuscarinic agent used to counteract extrapyramidal symptoms (EPS) induced by antipsychotics. This combination is specifically formulated to manage psychotic disorders while mitigating the risk of drug-induced parkinsonism and other EPS. The FDC is widely used in the Indian context for its convenience and improved adherence, though its use as a prophylactic combination is debated among experts.
Adult: Initial: One tablet (Risperidone 3mg + Trihexyphenidyl 2mg) once daily, preferably in the evening. Titration: Based on clinical response and tolerability. Risperidone dose range is typically 2-6 mg/day. The fixed Trihexyphenidyl 2mg dose is not titratable separately.
Note: Administer orally with or without food. Swallow whole with a glass of water. Do not crush or chew. Evening administration is preferred to minimize daytime sedation and orthostatic hypotension. Avoid abrupt discontinuation.
Risperidone exerts its antipsychotic effect through a high-affinity antagonism of dopamine D2 and serotonin 5-HT2A receptors in the mesolimbic pathway. Its higher 5-HT2A/D2 ratio is associated with lower extrapyramidal symptoms at therapeutic doses. Trihexyphenidyl is a competitive antagonist at central muscarinic acetylcholine receptors (M1), which helps counteract the dopamine blockade-induced excessive cholinergic activity in the nigrostriatal pathway, thereby reducing EPS like dystonia, akathisia, and parkinsonism.
Pregnancy: Pregnancy Category C (US FDA). Risperidone: Limited human data, potential risk. Trihexyphenidyl: Data insufficient. Use only if potential benefit justifies potential fetal risk. Neonates exposed in 3rd trimester are at risk for extrapyramidal symptoms or withdrawal. Register with pregnancy registry if used.
Driving: May impair alertness, motor coordination, and reaction time. Patients should not drive or operate heavy machinery until their individual response is known, especially during initial treatment and dose adjustments.
| Other CNS Depressants (Alcohol, Benzodiazepines, Opioids) | Additive sedation, respiratory depression, impaired psychomotor function. | Major |
| Other Anticholinergics (Tricyclic Antidepressants, Antihistamines, Oxybutynin) | Additive anticholinergic effects: toxic delirium, hyperthermia, constipation, urinary retention. | Major |
| Levodopa, Dopamine Agonists | Trihexyphenidyl may enhance effects; Risperidone antagonizes dopaminergic effects, reducing efficacy of levodopa. | Major |
| CYP2D6 Inhibitors (Fluoxetine, Paroxetine, Quinidine) | Increase risperidone plasma levels, increasing risk of side effects. | Moderate |
| CYP3A4 Inducers (Carbamazepine, Rifampicin, St. John's Wort) | Decrease risperidone plasma levels, potentially reducing efficacy. | Moderate |
| Antihypertensives | Enhanced hypotensive effect due to risperidone's alpha-blockade. | Moderate |
| QT-prolonging drugs (Class Ia/III antiarrhythmics, Macrolides, Fluoroquinolones) | Additive risk of QT prolongation and torsades de pointes. | Major |
Same composition (Risperidone (3mg) + Trihexyphenidyl (2mg)), different brands: