Pfizer will trial its "Aricept" donepezil hydrochloride drug, commonly used to treat some of the symptoms of Alzheimer's disease, on people between the ages of 15 and 39 who have Down's syndrome.
Down's is a congenital disorder frequently characterised by diminished mental faculty and physical abnormalities, including distinctive facial features.
It is caused by a chromosome defect and is proportionately more common in people born of older mothers.
From the mid-teens onwards, roughly six percent of people with Down's syndrome lose physical co-ordination, experience sleep disorders and become more withdrawn, a spokesman for pharmaceutical company Eisai said.
"This testing, if effective, might show that the medication could improve their condition and help improve the quality of their lives," the spokesman said.
It could also ease the burden on their caregivers, he added.
Some of the symptoms experienced by people with Down's syndrome are common to some sufferers of Alzheimer's, for which Aricept has long been prescribed.
The testing will initially involve 10 hospitals across Japan, with dozens of people in the affected age group.
The programme will continue for up to four years, the spokesman said.
The drug is not intended to treat the underlying condition, he said, rather it is hoped it will target some of the symptoms.
Around one in every 830 newborns in the United States has Down's syndrome, according to the National Institutes of Health. This equates to about 250,000 people in the US population, it says.