Which antipsychotic medication is effective for PD psychosis but is rarely used due to serious side effects?

Clozapine is considered effective for treating Parkinson’s disease (PD) psychosis due to its unique pharmacological profile that can mitigate psychotic symptoms in patients with Parkinson's. It is an atypical antipsychotic that acts primarily as a serotonin 5-HT2A receptor antagonist, which is beneficial in treating treatment-resistant schizophrenia as well as psychotic symptoms associated with PD.

However, clozapine is rarely used for PD psychosis primarily because of its potential for serious side effects, including agranulocytosis, seizures, myocarditis, and cardiomyopathy, which can severely limit its safety profile. Regular monitoring of white blood cell counts is required for patients on clozapine to minimize the risk of agranulocytosis, making its use less practical in many clinical settings. This risk-benefit ratio limits its use primarily to patients who are treatment-resistant or cannot tolerate other medications.

In contrast, while other antipsychotic medications listed in the choices may alleviate symptoms of PD psychosis, they don't carry the same level of effectiveness or the constellation of severe side effects that come with clozapine.