Inclusion and exclusion criteria

Participant inclusion criteria

1. Adults aged over 18 years
2. Meet MDS criteria for Parkinson’s disease or Revised criteria for Lewy Body Dementia
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 from the time consent is signed until 6 weeks after treatment discontinuation and inform the trial team if pregnancy occurs. For the purpose of clarity, true abstinence is when this is in line with the preferred and usual lifestyle of the subject.
9. Females of childbearing potential have a negative urine pregnancy test within 7 days prior to being registered/randomised.
10. If treated with an antipsychotic at the time of enrolment, can still participate in the trial provided the antipsychotic drug is stopped the day before the trial medication is commenced.
11. Sufficient knowledge of the English language to be able understand the participant information sheet and trial materials including outcome assessments. 

Participant exclusion criteria 

1. Bradycardia (<50 bpm)
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
5. Prescribed anti-emetics of the same class (tropisetron, granisetron, dolasetron)
6. History of hypersensitivity to ondansetron, it’s excipients
7. Clinically significant co-morbidity, which in the opinion of the PI would prevent safe participation in this study
8. Unable or unwilling to comply with study procedures.
9. Participation in another CT-IMP in the previous 28 days.