Los Angeles - Women who cannot tolerate side-effects of hormone replacement therapy might be able to derive the same benefits with a half dose, a new study suggested yesterday. Researchers at the University of California found post-menopausal woman given half the normal dose of Estratab, a plant-based estrogen, showed improved bone density. They did not, however, have increased vaginal bleeding, a precursor to endometrial cancer, which is associated with higher doses of animal-based estrogen.