June 15 (HealthDay News) -- Depression might increase the chance of developing Alzheimer's disease among those already experiencing memory problems, a new study says.

But the drug donepezil (Aricept), commonly prescribed for people with Alzheimer's, could slow the depressed person's slide into the disease, the study also found.

The findings are reported in the June 16 issue of Neurology.

"Our longer-term findings add to the body of evidence that suggests depression is a major risk factor for Alzheimer's disease," study author Po H. Lu, an assistant professor of neurology at the David Geffen School of Medicine at the University of California, Los Angeles, said in a news release from the American Academy of Neurology.

The researchers' conclusions stemmed from a three-year study of 756 middle-aged and older people diagnosed with mild cognitive impairment, in which the memory is worse than would be expected at a given age but who otherwise show no signs of Alzheimer's disease.

Based on a test to measure depressive symptoms, the researchers found that the more depressed people were, the greater their chance of developing Alzheimer's.

During the study period, participants took either vitamin E, donepezil or a placebo. After about two years, 14 percent of those diagnosed with depression who took donepezil had developed Alzheimer's, compared with 29 percent of those who took either the vitamin or placebo. Donepezil had little effect in the group of people who were not depressed, according to the study.

"If we can delay the progression of this disease for even two years, it could significantly improve the quality of life for many people dealing with memory loss," Lu said.

Donepezil can help control symptoms of people with mild to severe Alzheimer's disease, but it is not approved in the United States as a treatment for mild cognitive impairment.

SOURCE: American Academy of Neurology, news release, June 15, 2009