Remacemide is being developed in clinical trials by the pharmaceutical company AstraZeneca to treat people with epilepsy.
However, researchers have also discovered that the drug is also safe for use by people with Parkinson's disease who are currently taking other drugs to treat their condition.
The researchers, whose work is published in Neurology, the journal of the American Academy of Neurology, say remacemide does not work when taken in isolation.
However, when added to other drugs such as levodopa, it reduces symptoms and may slow the progress of Parkinson's.
Lead researcher Dr Steven Schwid, a neurologist at the University of Rochester in New York, said: "Our primary goal was to learn how to use the drug safely.
"If ongoing studies confirm that remacemide used in conjunction with dopaminergic (drug) therapy improves patients' symptoms, it may be the first of a new class of Parkinson's therapies.
"If it is also proved to have neuroprotective qualities by preventing the progression of the disease, it could be an even more significant advance in the treatment of Parkinson's disease."
Experimental tests
Dr Schwid's team tested 200 patients with early Parkinson's who were not taking levodopa or other dopamine drugs.
Nearly all were able to take the drugs. Some reported nausea and dizziness, but nobody suffered serious side effects.
Parkinson's is an incurable, fatal disorder. Patients initially suffer from trembling or muscle freezing, and eventually lose the ability to move at all. Some develop dementia.
At present, drugs to treat Parkinson's only work for a few years.
It is thought that Parkinson's is linked to the death of a certain type of brain cell that produces a chemical called dopamine.
Dopamine effectively ferries messages around the brain, and is linked to the co-ordination of body movement. As levels of the chemical drop, co-ordination begins to falter.
It is also suspected that brain cell death in Parkinson's patients is caused by over-activity of another brain chemical called glutamate.
Remacemide interferes with glutamate.
A spokesman for the Parkinson's Disease Society said many drugs could help to control the symptoms of the disease, but a drug which actually slowed its progression would be a significant breakthrough.
He said: "We would welcome any drug that helps to improve the quality of life for people with Parkinson's disease, but a lot more research is required."
The drug is also being tested against Huntington's disease, an inherited condition in which the brain slowly degenerates.