The hormone estriol, produced during pregnancy, is frequently compounded into bioidentical preparations in the United States. While some think it to be a weaker estrogen, with a more limited period of effectiveness than estradiol, it has been demonstrated to be a stronger estrogen in certain ways. Though initial research in the 1970s suggested possible use, follow-up studies have failed to confirm this potential. Estriol is not found in any FDA-approved drug, and its safety and effectiveness as a hormone supplement is unknown.