Study Goal:

Hallucinations are a symptom of Parkinson's that make it tricky to know what is real. Hallucinations are very common and sometimes very troublesome in Parkinson's, especially in older people and those who have had Parkinson's for a long time. There are only a  few effective and safe treatments for people with Parkinson’s experiencing hallucinations. Finding safer options is a priority for patients, their families and doctors. This study aims to test a safe drug to see if it can help people with Parkinson's who have hallucinations.

What's Involved:

This trial tests whether a drug called ondansetron helps people with Parkinson's who are experiencing hallucinations. Ondansetron is a safe drug which is normally used to treat sickness after operations. Participants will either take ondansetron or a placebo (a "fake" sugar pill that looks identical but contains no drug). Study visits will take place at the start of the study and at 6 weeks, 12 weeks, 18 weeks and 24 weeks. The study team will measure any changes in hallucinations, delusions (false beliefs), Parkinson's symptoms (such as shaking, anxiety, sleep quality), memory and quality of life.

Who can take part? 

People who:

• Have a diagnosis of Parkinson’s or related condition, Lewy Body Dementia 

• Are 18 years or older 

• Experience visual hallucinations at least every week

• Hallucinations remain after treatment  

• People can also have dementia; arrangements can be made if they are not able to make the decision about whether to take part themselves.

Why was the trial paused?

Following advice from the independent Data Monitoring and Ethics Committee, recruitment was paused to review safety data after the first 100 participants had taken part. The committee thought there were more safety issues in those who received ondansetron than the placebo. The study team investigated these concerns carefully to ensure the safety of participants, however they found that data recording errors were responsible. There were no concerns over the safety of ondansetron in this trail. Recruitment is now back underway and has been extended until July 2026.