Trial of Ondansetron as a Parkinson's HAllucinations Treatment

1. Adults aged over 18 years.

2. Meet MDS criteria for Parkinson's disease or revised criteria for DLB.

3. Score of 3 or more on the SAPS-H visual hallucinations item, indicating the presence of visual hallucinations at least weekly in the previous month.

4. Score of 3 or more on SAPS-H global rating, indicating moderate symptom severity.

5. Score of 4 or more on CGI-S, indicating moderate symptom severity.

6. On a stable dose of anti-Parkinson's medication, cholinesterase inhibitor or memantine for at least 28 days.

7. Capacity to give informed consent or, if lacking, legal representative able to give consent.

8. Pre-menopausal women, and men whose partners are of child bearing potential will agree to use effective contraception. 9) If treated with an antipsychotic drug at the time of enrolment, can still participate, provided the drug is stopped the day before trial medication is commenced.

1. Bradycardia (<50 bpm) (rescreen if reversible).

2. Congenital long QTc syndrome or presence of clinically significant prolongation of QTc (>460 ms for men or >470 ms for women) on ECG screening.

3. Severe hepatic failure (bilirubin >50 micromole/L)

4. Prescribed apomorphine (if apomorphine is discontinued, rescreen once stable on an alternative anti-Parkinson's treatment).

5. Prescribed tropisetron, granisetron, dolasetron.

6. History of hypersensitivity to ondansetron and its excipients (or those of placebo) or drugs listed in 5).

7. Participation in another Clinical Trial of an Investigational Medicinal Product (IMP) in the previous 28 days.