Hormone replacement therapy may prevent Alzheimer's disease in women, but the benefit exists only when the therapy lasts 10 years or more, according to a report published in the Journal of the American Medical Association.

No protective benefit was seen from hormone replacement therapy that had not begun at least several years prior to the onset of the disease.

"Our findings, along with other recent work, suggest that hormone replacement therapy may be effective for the primary prevention of Alzheimer's disease -- if not for its treatment," the researchers reported.

Researchers compared the rates of Alzheimer's disease between 1995 and 2000 in 1,357 elderly men and 1,889 elderly women living in Cache County, Utah.

The women who had taken hormone replacement therapy for at least ten years were 2.5 times less likely than women who had never used hormone replacement therapy to develop Alzheimer's disease. This lower rate among long-term users of hormone replacement therapy was comparable to the rate found in the men.

"A new finding in this study is an apparent limited window of time during which sustained hormone replacement therapy exposure seems to reduce the risk of Alzheimer's disease," wrote the researchers. "We found that, in contrast with (use earlier in life), hormone replacement therapy exposures within ten years of Alzheimer's onset yielded little, if any, apparent benefit."

Researchers speculate that estrogen may exert a protective effect against Alzheimer's only before extensive damage occurs in the brain.

The study also found no preventive effect from use of calcium supplements and multivitamins.

Other sources: Journal of the American Medical Association