Estradiol (medication)
Estradiol is available in a variety of different formulations, including oral, intranasal, transdermal/topical, vaginal, injectable, and implantable preparations. An ester may be attached to one or both of the hydroxyl groups of estradiol to improve its oral bioavailability and/or duration of action with injection. Such modifications give rise to forms such as estradiol acetate (oral and vaginal), estradiol valerate (oral and injectable), estradiol cypionate (injectable), estradiol benzoate (injectable), estradiol undecylate (injectable), and polyestradiol phosphate (injectable; a polymerized ester of estradiol), which are all prodrugs of estradiol.
Estriol (medication)
Estriol is a far less potent estrogen than is estradiol, and is a relatively weak and atypical estrogen. Given by subcutaneous injection in mice, estradiol is about 10-fold more potent than estrone and about 100-fold more potent than estriol. In most tissues, such as the liver and endometrium, estriol is weakly estrogenic, but in the vaginal epithelium, it produces pronounced and full estrogenic responses. The medication has been found to reduce hot flashes, improve vaginal atrophy, reverse the postmenopausal decline in skin thickness and collagen content, suppress gonadotropin secretion, and produce proliferation of breast epithelium. Conversely, estriol does not consistently affect bone resorption or fracture risk, does not increase breast density or the risk of breast cancer, and, at oral dosages of 2 to 4 mg/day, does not affect liver proteins, lipid metabolism, or hemostatic parameters. Additionally, vaginal estriol does not appear to produce endometrial proliferation or increase the risk of endometrial hyperplasia, and some studies have found this to be the case for oral estriol as well. On the other hand, it appears that estriol may be able to stimulate the growth of active breast cancer. Via the oral route, estriol has approximately 30% of the potency of estradiol on hot flashes relief and suppression of follicle-stimulating hormone secretion and about 20% of the potency of estradiol on stimulation of liver production of high-density lipoprotein (HDL) cholesterol. In relation to its differing effects, estriol can be considered a safer estrogen than estradiol.