Progesterone is often given with estrogen in hormone replacement therapy. Previous studies have suggested that estrogen offers women some protection against Alzheimer's disease. USC Davis School of Gerontology professor Christian Pike, wondered if the same could be true of progesterone.

Pike and colleagues tested progesterone on female mice which were genetically predisposed to develop an Alzheimer's-like disease. The mice showed symptoms within months. Treatment with estrogen reversed the symptoms, treatment with progesterone did not.

When the two hormones were given together, progesterone appeared to hinder estrogen's main beneficial function: preventing the buildup of beta amyloid protein, the key risk factor in Alzheimer's. "Estrogen no longer decreases the amount of beta amyloid" when progesterone is present, Pike said.

Progesterone's effects were not all bad, Pike added. The hormone appeared to inhibit tau hyperphosphorylation, another chemical process implicated in Alzheimer's.

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