Gerontologist Christian Pike of the University of Southern California said progesterone is given with estrogen in hormone replacement therapy. Previous studies have suggested that estrogen offers women some protection against Alzheimer's disease.

However, the study published in the Journal of Neuroscience, found progesterone has limited benefit for mice with Alzheimer's symptoms when taken alone.

When taken with estrogen, progesterone inhibits some of estrogen's beneficial effects, the study found.

The problem is not necessarily progesterone itself, it could be the dosage, Pike explained. In the study, progesterone was given in a daily dose, which fails to replicate the premenopausal body's natural cycles of hormone production.

"This is probably not the best way to be delivering progesterone," Pike said in a statement. "Giving a constant dose of progesterone appears to antagonize a lot of the beneficial effects of estrogen."