This study will determine if ondansetron, a drug already used in the NHS, can be used as a treatment for visual hallucinations in people with Parkinson’s Disease or Lewy Body Dementia.

Visual hallucinations can be a common and distressing feature of Parkinson’s Disease and can have a significant impact on quality of life.

The Trial of Ondansetron as a Parkinson’s HAllucinations Treatment or TOP HAT is being carried out by researchers at the Northern Care Alliance on behalf of a team at UCL (University College London).


  • Aged 18+
  • Diagnosed with Parkinson’s Disease (on stable medication) or Lewy Body Dementia
  • Experience visual hallucinations


  • Salford Royal Hospital or Fairfield General Infirmary


Taking part involves:

  • Attending an initial screening appointment (online/telephone)
  • Attending two clinic appointments, which will include a range of tests (eg blood and urine samples, liver function, blood pressure, electrocardiogram, and body measurements) as well as completing a short questionnaire. (approx. 1 hour each)
  • Attending four online/telephone appointments to assess progress, side effects, symptoms etc
  • Taking the study drug for 12 weeks *

* Participants will be randomly selected to receive either the study drug (ondansetron) or a placebo


Transport and lunch will be provided for clinic visits.


Recruiting until

  • 14 July 2024

Trial ID

  • 266504

Want to take part or learn more?

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