Parkinson's drugs can cause drowsiness
The New York based team documented the cases of eight patients who had car accidents after taking the "dopamine agonist" drugs to relieve the disabling brain condition.
Four of these, said the study, published in "Neurology", had also fallen asleep without warning during business meetings or even half way through telephone calls.
All the patients were taking Mirapex, made by Pharmacia and Upjohn and known generically as pramipexole, or SmithKline Beecham's ropinirole, sold under the name Requip.
When they stopped taking the drugs the sleep attacks stopped.
Further investigations
Pharmacia and Upjohn stress that there have been very few incidences, and that the drug carries a standard drowsiness warning.
They issued a statement which said: "We are actively investigating these events to determine more details, inlcuding other medications the patients may have been taking, their medical histories, and so on."
They stressed that the drugs may not necessarily have caused the patients to fall asleep.
But they go on: "All marketed drugs in the dopamine agonist class list somnolence or drowsiness as a potential adverse event.
"Patients should be advised that until they know how Mirapexin will affect them, they should not drive or operate potentially dangerous machinery."
Dr Alistair Benbow, medical director of SmithKline Beecham, said that ropinirole was associated with sleepiness, but only in a tiny minority of patients, and that the company had never previously come across patients who experienced sudden sleep attacks.
However, he said that the drug did carry a warning about the possible dangers of driving.
Dopamine agonists help the brain to react better to dopamine, an important chemical which aids movement.
In Parkinson's the part of the brain that produces dopamine is slowly destroyed.
There is no cure, but drugs that make dopamine or make the body more sensitive to its effect can help reduce the symptoms for several years.
Parkinson's causes symptoms ranging from slight tremors to paralysis, and eventually death.