risperidone

Risperidone blocks D2 dopamine receptors (reducing positive psychotic symptoms) and 5-HT2A serotonin receptors (which helps reduce extrapyramidal side effects and may improve negative symptoms compared to first-generation antipsychotics). At low doses, its serotonin antagonism dominates, but at higher doses, D2 antagonism becomes more prominent, increasing the risk of extrapyramidal symptoms (EPS) and hyperprolactinemia—risperidone has the highest prolactin-elevating effect among atypical antipsychotics due to poor blood-brain barrier penetration allowing peripheral D2 blockade in the pituitary.

Clinical uses include schizophrenia, acute mania/bipolar disorder, and irritability in autism spectrum disorder (FDA-approved for children). It is also used off-label for Tourette syndrome and severe behavioral disturbances in dementia (with caution due to increased mortality risk in elderly patients with dementia-related psychosis - black box warning).

Key adverse effects: hyperprolactinemia (galactorrhea, gynecomastia, amenorrhea, sexual dysfunction), EPS (more than other atypicals, less than typicals), weight gain, metabolic syndrome (moderate risk, less than olanzapine/clozapine), orthostatic hypotension, and QT prolongation. Rare but serious: neuroleptic malignant syndrome (NMS) and tardive dyskinesia.

Mechanistically, it's classified as a benzisoxazole derivative. Its active metabolite, paliperidone (9-hydroxyrisperidone), is also marketed separately as an antipsychotic. Available in oral and long-acting injectable (LAI) formulations, useful for improving medication adherence.

High-yield board points: distinguishing risperidone's higher prolactin/EPS profile from other atypicals like quetiapine (low EPS, sedating) or clozapine (agranulocytosis risk, reserved for treatment-resistant schizophrenia).