Pimavanserin vs. Quetiapine for Parkinson's Psychosis

Inclusion Criteria: * Veteran * Diagnosis of Parkinson's Disease consistent with UK Parkinson's Disease Society Brain Bank Clinical Diagnostic Criteria * Psychosis \[with Neuropsychiatric Inventory (NPI) hallucinations (B) or delusions (A) score 4 or greater\] * Stable dose of PD medications for at least 2 weeks * If on an acetylcholinesterase inhibitor (AChEI) initially prescribed at least 3 months prior and stable dose (no dose or medication change) for past month * Informed other must provide informed consent and agree to attend all study visits. The informed other must be at least 18 years of age and have regular contact with the patient (on average at least 4 days per week and at least 2 hours per day, or at least 3 days per week and at least 4 hours per day, that is with patient) via in-person, video, or telephone * English-speaking INFORMED OTHER * Age 18 years or older * Must have regular contact with the patient (on average at least 4 days per week, and at least 2 hours per day, or at least 3 days per week and at least 4 hours pr day, that is with patient) via in-person, video, or telephone * Agree to attend all study visits * Be able to provide informed consent * English-speaking Exclusion Criteria: * Psychosis symptoms severe enough to preclude enrollment in a clinical trial and require prompt clinical care instead * Treatment with quetiapine \>50 mg/day or pimavanserin in the past 3 months, or quetiapine 50 mg/day or another antipsychotic in the past week prior to study randomization * Deep brain stimulation (DBS) surgery within 3 months or has had stimulator adjustments in the previous 2 weeks * History of a psychotic disorder prior to PD, including bipolar disorder, schizophrenia, schizoaffective disorder, and major depressive disorder with psychotic features, if it is thought to be the cause of the current psychosis symptoms * Suspected atypical parkinsonian disorder or dementia with Lewy bodies (DLB) * Psychosis secondary to other toxic or metabolic disorder * History of long QT syndrome * Documented chart evidence indicating persistent hypoglycemia, hypokalemia, hypomagnesemia that would put patient at increased risk for QTc prolongation. * History of ventricular arrhythmias, except when treated with an implantable cardioverter defibrillator (ICD) or pacemaker, or untreated or unstable atrial fibrillation/flutter * Currently taking medications that are moderate or strong CYP3A4 inducers or strong CYP3A4 inhibitors * Concomitant use of drugs that prolong the QTc interval with a known risk of Torsades de Pointes * Comorbid medical condition determined too severe by Site Investigator to allow participation in clinical trial * Failure to tolerate quetiapine or pimavanserin previously * Severe cognitive impairment (MoCA score \<5) * Nursing home placement at screening or planned placement during the study, unless approved by study Co-Chairs. Approval will depend upon nursing facility agreement to receive, return, and administer medications or allow participant to self-administer study medications; appropriate IO availability; and transportation availability for study visits. * Currently enrolled in another therapeutic or interventional study * Pregnant, or a female of child-bearing potential who is unwilling to use a reliable form of contraception