The expanded approval makes Exelon the first drug for the treatment of the chronic loss or impairment of intellectual capacity in Parkinson's disease patients, the Food and Drug Administration said.
The drug, also called rivastigmine tartrate, is made by Switzerland-based Novartis AG.
The FDA originally approved Exelon in 2000 for Alzheimer's patients with dementia.
About 0.2 percent to 0.5 percent of people 65 and older are affected by Parkinson's dementia and experience impairments in executive function, memory and attention, the FDA said.
Parkinson's patients treated with the drug in a 24-week trial scored higher on a scale that measures mental processes than did those given dummy pills, the FDA said.
Patients on the drug can suffer significant gastrointestinal reactions. In clinical trials, nearly half the patients developed nausea. Many also lost significant amounts of weight. And some patients treated with Exelon experienced a worsening of the tremors associated with the disease, according to the FDA.