Estradiol (medication)
Estradiol is also available in the U.S. in combination with progestogens for the treatment of menopausal symptoms and as a combined hormonal contraceptive:Estradiol and estradiol esters are also available in custom preparations from compounding pharmacies in the U.S. This includes subcutaneous pellet implants, which are not available in the United States as FDA-approved pharmaceutical drugs. In addition, topical creams that contain estradiol are generally regulated as cosmetics rather than as drugs in the U.S. and hence are also sold over-the-counter and may be purchased without a prescription on the Internet.
Estriol
Although estriol is an efficacious agonist of the ERs, it is reported to have mixed agonist–antagonist (partial agonist) activity at the ER; on its own, it is weakly estrogenic, but in the presence estradiol, it is antiestrogenic. Given by subcutaneous injection in mice, estradiol is about 10-fold more potent than estrone and about 100-fold more potent than estriol. It is notable that unlike estriol, estrone can be metabolized into estradiol, and most of its potency "in vivo" is in fact actually due to conversion into estradiol.