A popular Alzheimer’s drug appears to delay the onset of the disease among patients with memory loss although does not prevent it, researchers announced on Sunday.

Aricept reduced the risk of developing Alzheimer’s among patients diagnosed with mild cognitive impairment (MCI), a memory disorder that is often a precursor to the brain disease, according to research released at the 9th International Conference on Alzheimer’s Disease and Related Disorders.

But the risk reduction lasted for only 18 months of a three-year trial, said Dr. Ronald Petersen of the Mayo Clinic in Rochester, Minnesota, who led the study of 769 people. When patients taking Aricept developed Alzheimer’s they did so about six months later than those taking a placebo.

“This is the first study to demonstrate a positive treatment effect on progression to Alzheimer’s disease from MCI,” Petersen said.

“It looks like the drug had a modest, time-limited effect. Nonetheless, we are optimistic because we have begun to make progress toward delaying the development of Alzheimer’s.”

The study contradicts one published in the Lancet medical journal in June showing Aricept had no beneficial effect.

Japanese drugmaker Eisai Co Ltd co-markets with Aricept, known generically as donezapil, with U.S. drug maker Pfizer Inc.

Alzheimer’s, an incurable and usually fatal disease, impairs the ability to remember, learn, think and communicate. It affects an estimated 4.5 million Americans, a number that is expected to increase to as many as 16 million by 2050.

Former President Ronald Reagan died with Alzheimer’s last month and his widow Nancy has called for increased government funding for research to find ways to prevent or cure it.

The move from normal age-related forgetfulness to mild cognitive impairment can be recognized when people start forgetting information such as appointments or social events, Petersen said.

“They are forgetting information that they used to remember easily,” Petersen said at a news conference.

In another study, Dr. Frederick Unverzagt of the Indiana University School of Medicine studied an illness similar to MCI, known as Cognitive Impairment No Dementia.

The study found that some but not all the patients with this condition were at the earliest stages of Alzheimer’s, and that the risk of developing CIND was higher among those with a history of Head injury and those who grew up in rural areas.