TOP HAT: Trial of a licensed drug as a Parkinson's Hallucination Treatment
Research teams across the UK are investigating if ondansetron has a meaningful treatment effect on Parkinson's and Lewy Body Dementia hallucinations.
As this is an already licensed low-cost drug, if the research shows significant outcomes, ondansetron could be available for people with Parkinson's who experience hallucinations within the next few years.
306 people diagnosed with Parkinson's or Lewy Body Dementia (LBD) who:
• experience visual hallucinations at least once a week,
• are not taking apomorphine or any anti-psychotic medication,
• and have been taking a stable dose of Parkinson's or LBD medication for 28 days before starting the research
The first activity is a telephone call to discuss the research more fully, and complete some questionnaires. This will be followed by a brief face-to-face visit to assess your Parkinson’s, take a blood sample, and monitor your heart rhythm.
If you are eligible to take part, you will either receive ondansetron or dummy medication tablets for 12 weeks, which will be shipped to you via a courier. The research team will maintain regular contact throughout to take assessments and let you know how much medication to take. This contact will be through telephone, video call, and face-to-face.
3 face-to-face visits are required and, for some research sites, home visits may be an option.
For more information, please read the Participant Information Sheet or contact the TOP HAT team by email at [email protected].
Interested in taking part?
Find your nearest research site below and email the TOP HAT team at [email protected]
The deadline for taking part in this research is 1 July 2026.
Maria and Desmond's experience of taking part in the TOP HAT trial
“I’d really encourage anyone thinking about it to take part. It was so easy, just one more pill. And the research visits are just the same as the routine appointments. It was like having extra consultations, we felt like VIPs!” - Maria & Desmond
Parkinson's UK
Belfast Health and Social Care Trust, Room 9, 4th Floor, Dunluce Heath Centre, 1 Dunluce Ave
Belfast
BT9 7HR
United Kingdom
Queen Elizabeth University Hospital
Clinical Trials Unit
1345 Govan Road
Glasgow
G51 4TF
United Kingdom
Salford Royal NHS Foundation Trust
Stott Lane
Salford
M6 8HD
United Kingdom
Pennine Acute Hospitals NHS Trust, Ward 19 Clinical Research Unit, Fairfield General Hospital
Bury
BL9 7TD
United Kingdom
North Tyneside Hosptial
Rake Lane
North Shields
NE29 8NH
United Kingdom
Sherwood Forest Hospitals NHS Foundation Trust
Kings Mill Hospital
Sutton In Ashfield
NG17 4JL
United Kingdom
Queen Elizabeth Hospital
Neurology Department
Stadium Road
London
SE18 4QH
United Kingdom
The Shrewsbury and Telford Hospital NHS Trust, Research & Innovation Department, The Royal Shrewsbury Hospital, Mytton Oak Road,
Shrewsbury
SY3 8XQ
United Kingdom
Queen's Hospital
Barking, Havering and Redbridge University Hospitals NHS Trust
Neuro Research office, Neurophysiology Department, Orange Zone, Ground Floor
Romford
RM7 0AG
United Kingdom
Research and Development Office, Chertsey House, St Peter's Hospital, Guilford Lane,
Chertsey
KT16 0PZ
United Kingdom
Luton and Dunstable NHS Foundation Trust
Ground Floor Nova House
Lewsey Road
Luton
LU4 0DZ
United Kingdom
Christchurch Hosptial
Fairmile Road
Christchurch
BH23 2JX
United Kingdom
Coventry and Warwickshire Partnership NHS Trust, Wayside Business Park, Wayside House, Wilsons Lane,
Coventry
CV6 6NY
United Kingdom
Southern Heath Memory Assessment and Research Centre (MARC), Tom Rudd Unit, Moorgreen Hospital, Botley Road, West End,
Southampton
SO30 3JB
United Kingdom
Research Department, 4th floor Academic Centre, University Hospital of Llandough
Penarth
CF64 2XX
United Kingdom
Leeds and York Partnership NHS Foundation Trust
Research and Development, 1st Floor North Wing, St Mary’s House, St Mary’s Road
Leeds
LS7 3JX
United Kingdom
Neuroprogressive and Dementia Network, Old Pharmacy Building, Western General Hospital, Crewe Road South
Edinburgh
EH4 2UX
United Kingdom
Clinical Trials Office, Ward 25, University Hospital Monklands, Monkscourt Avenue,
Airdrie
ML6 0JS
United Kingdom
Research & Development, Two Bridges, Guildford Street,
Chertsey
KT16 9AU
United Kingdom
University College London Hospital
Leonard Wolfson Experimental Neurology Centre CRF National Hospital for Neurology and Neurosurgery
Queen Square
London
WC1N 3BG
United Kingdom
Royal Berkshire Hospital
London Road
Reading
RG1 5AN
United Kingdom