Trial of Ondansetron as a Parkinson’s Hallucinations Treatment


TOPHAT is a double-blind, randomised, placebo-controlled, flexible-dose trial of ondansetron as a treatment for Parkinson’s hallucination, with a 12-week primary outcome and follow-up to 24 weeks. 


The principle aim is to answer the question: ‘Will ondansetron be better than placebo at reducing the frequency and/or severity of visual hallucinations over 12 weeks’?  

We will measure this using Scale for Assessment of Positive Symptoms-Hallucinations (SAPS-H) scores.

Secondary aims will answer the following questions:

  • ‘Will ondansetron reduce delusions’? This information is important as it is possible that ondansetron’s effects will extend beyond hallucinations
  • ‘Will ondansetron be safe and well tolerated in people with Parkinson’s? Side effects were reported in the small studies (headache, constipation) but nobody stopped taking medication. We also wish to establish that ondansetron will not worsen motor, non-motor or cognitive symptoms of Parkinson’s.
  • ‘Will ondansetron be cost effective for NHS use’? We will be collecting information on health related quality of life and health and social care resource use