The drug, which is also marketed by Pfizer, is already approved to treat mild to moderate dementia in patients of Alzheimer's, which affects roughly 4.5 million mostly older Americans.

There is no cure for the condition, typified by memory loss and other cognitive impairment that can quickly worsen.

Alzheimer's is the most common form of dementia.

Eisai, based in Japan, earlier this month cited growing US sales of the drug thanks to Medicare's new prescription drug benefit for the elderly and disabled.

It has forecast total Aricept sales at $1.9 billion, for the fiscal year through March 2007. Some analysts have said that estimate is conservative, but company officials have declined to comment.