A type of diabetes drug may offer a glimmer of hope in the fight against Parkinson's disease, research in the journal Plos Medicine suggests.
Scientists found people taking glitazone pills were less likely to develop Parkinson's than patients on other diabetes drugs.
But they caution the drugs can have serious side-effects and should not be given to healthy people.
Instead, they suggest the findings should prompt further research.
There are an estimated 127,000 people in the UK with Parkinson's disease, which can lead to tremor, slow movement and stiff muscles.
And charities say with no drugs yet proven to treat the condition, much more work is needed in this area.
The latest study focuses solely on people with diabetes who did not have Parkinson's disease at the beginning of the project.
Researchers scoured UK electronic health records to compare 44,597 people prescribed glitazone pills with 120,373 people using other anti-diabetic treatment.
They matched participants to ensure their age and stage of diabetes treatment were similar.
Scientists found fewer people developed Parkinson's in the glitazone group - but the drug did not have a long-lasting benefit. Any potential protection disappeared once patients switched to another type of pill.
Dr Ian Douglas, lead researcher at the London School of Hygiene and Tropical Medicine, said: "We often hear about negative side-effects associated with medications, but sometimes there can also be unintended beneficial effects.
"Our findings provide unique evidence that we hope will drive further investigation into potential drug treatments for Parkinson's disease."
He suggests such therapies would be most useful in the earliest stages of the disease when there is little damage to nerves.
But as glitazone drugs have previously been linked to serious heart and bladder problems, scientists caution that healthy people should not take the drugs.
Dr Arthur Roach, from the charity Parkinson's UK, added: "Hopefully the results of this study will spark further research into developing drugs that work in a similar way to glitazone drugs, and have the ability to reduce someone's chance of developing Parkinson's."