A drug has been found to significantly slow the progression of Parkinson's disease.
Parkinson's is linked to brain chemicals
Scientists compared the effect of the drug, ropinirole, with the widely used treatment levodopa.
They found that ropinirole was more effective at slowing down the loss of nerve function associated with the early stages of Parkinson's.
It was also less likely to cause side effects.
However, it was not as effective as levodopa at controlling the symptoms of the disease, such as shaking, lack of coordination and frozen expression.
Parkinson's is caused by lack of a crucial brain chemical called dopamine.
Levodopa is converted in the brain into dopamine - thus helping to replenish stocks.
In contrast, Ropinirole works by stimulating the same receptors as dopamine.
Over a period of two years, scientists used a brain imaging technique to assess levels of nerve function in 186 patients with signs of early Parkinson's.
The images showed that on average the loss of nerve function was 35% slower in patients taking ropinirole.
They were also nine times less likely to develop the involuntary and uncontrollable jerking movements associated with the long-term use of levodopa.
Lead researcher Professor David Brooks, of the Medical Research Council Clinical Sciences Centre and Imperial College London, said: "Our work has shown that, compared with levodopa, prescribing ropinirole significantly slows the rate of nerve function loss in patients with Parkinson's disease.
"The findings are clearly important and we hope that future studies will show this slower loss of nerve function equates to long term benefits, offering the possibility of prolonged independence and quality of life to patients with Parkinson's disease."
More than four million people worldwide suffer from Parkinson's disease, making it the most common brain degenerative disease after Alzheimer's disease.
Ropinirole works by mimicking the action of the chemical dopamine by directly stimulating receptors in the brain.
Robert Meadowcroft, director of policy research & information at the Parkinson's Disease Society, described the results as "encouraging".
However, he said: "As the results also show that symptoms might not be as well controlled over a longer period of time, there is a need for greater research into this area to determine the longer term benefits of one treatment compared with the other."
The research is published in the journal Annals of Neurology.