Several doctors said the study is reassuring to women already on hormone therapy. There were 38 million women in 2010, according to IMS Health. Most prescriptions, 26 million, were for estrogen-only products, but 5.2 million were for progestin-estrogen products, and 5.3 million were for progestin-only products.
"These findings WILL change guidelines," Dr. Katherine Sherif, director of the Drexel Center for Women's Health, wrote in an email. "The pendulum is swinging back to the middle, partly due to women who benefit from hormones."
Wolf Utian, who founded and heads the National American Menopause Society, called the study the "foil to consistent negative statements" on hormone therapy, and emphasized the quality of life data in the study.
The preliminary findings are being announced in Orlando.
Dr. Lauren Hughes of the ABC News Medical Unit contributed to this report.