The results, published in the scientific journal The Lancet, show promise that exenatide and other similar drugs have potential as a new, future treatment for people with Parkinson's.
The trial of the drug at University College London involved 60 people with moderate Parkinson's.
Half received exenatide once a week for 48 weeks, while the other half (the placebo group) received a dummy injection.
At the end of the study, those who received exenatide had a slight but significant improvement in movement symptoms when measured off their regular Parkinson's medication.
In comparison, participants who received the placebo worsened slightly on this measure over the course of the study.
Importantly, the difference between the 2 groups was still there when they were re-assessed 12 weeks after treatment stopped.
However, there was no difference between the 2 groups on any other measures - including quality of life, non-motor symptoms or movement symptoms - while on their regular Parkinson's medication.
Throughout the study all participants were carefully and comprehensively monitored to assess any changes in their symptoms and to identify any potential safety issues or side effects.
Professor Tom Foltynie, the study's senior author, said:
"This is a very promising finding, as the drug holds potential to affect the course of the disease itself, and not merely the symptoms.
"With existing treatments, we can relieve most of the symptoms for some years, but the disease continues to worsen."
What's next for exenatide?
Professor David Dexter, Deputy Director of Research at Parkinson's UK, commented:
"These interesting results build upon an earlier, smaller trial and offer further encouragement that diabetes treatments could provide new treatments for Parkinson's in the future.
"The small benefits seen in this study are particularly promising because only a low level of exenatide actually reached the brain. This suggests that finding treatments that work in a similar way, but are better able to cross from the bloodstream into the brain, may be even more effective.
“Repurposing existing drugs in this way could allow treatments to be made available more quickly than starting from scratch to develop entirely new medication.
"No current treatment can slow or stop the progression of Parkinson's and this area warrants further research to deliver the breakthroughs people with Parkinson's urgently need."
Can I take exenatide now?
Before any new treatment can be made available to people it must be proven to be safe and effective in clinical trials.
Although these results show promise they don't provide sufficient evidence for exenatide to become an approved new treatment for Parkinson's at this stage.