A drug used to treat Alzheimer's disease should not be prescribed to people with milder mental impairment without first giving them a genetic test, a new study urges.

The drug is donepezil (brand name: Aricept).

Donepezil could speed mental decline in someone with mild cognitive impairment who has a specific genetic variation, according to Sophie Sokolow, an associate professor at the UCLA School of Nursing.

She and her colleagues found that patients with the K-variant of the butyrylcholinesterase (BChE) gene who took donepezil deteriorated faster than those who took a placebo.

Donepezil is approved in the United States to treat Alzheimer's disease but not mild cognitive impairment -- the stage between normal age-related decline and dementia. However, doctors often prescribe it "off-label" for patients with mild cognitive impairment, the study authors said.

For this study, the researchers examined data from a U.S. government-funded study published in 2005 that assessed donepezil as a possible treatment for mild cognitive impairment.

The findings reinforce the importance of physicians discussing the possible benefits and risks of donepezil with their patients, the researchers said in a university news release.

The study was published recently in the Journal of Alzheimer's Disease. Funding was provide by the U.S. National Institute on Aging.