Estrogen (medication)
Estradiol, estrone, and estriol have all been approved as pharmaceutical drugs and are used medically. Estetrol is currently under development for medical indications, but has not yet been approved in any country. A variety of synthetic estrogen esters, such as estradiol valerate, estradiol cypionate, estradiol acetate, estradiol benzoate, estradiol undecylate, and polyestradiol phosphate, are used clinically. The aforementioned compounds behave as prodrugs to estradiol, and are much longer-lasting in comparison when administered by intramuscular or subcutaneous injection. Esters of estrone and estriol also exist and are or have been used in clinical medicine, for example estrone sulfate (e.g., as estropipate), estriol succinate, and estriol glucuronide (as Emmenin and Progynon).
Bioidentical hormone replacement therapy
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.