Estriol belongs to natural hormone and is the metabolite of estradiol in vivo. It is mainly presented in the urine. Estrogen has a relative small activity with the oral activity being 6 times as high as estrone but being weaker than estradiol with non-carcinogenic effects. After its administration, the in vivo estradiol levels did not change. Estriol has selective action on the vaginal and cervical canal but has no effect on the uterus and endometrium entity. Animal experiments have shown that estriol has a stronger effect on vaginal epithelial keratosis than estradiol, thus being able to promote vaginal epithelial hyperplasia, superficial cells keratosis, mucosal angiogenesis and vaginal epithelial wound healing, but having a weak effect on the weight gain of mouse uterus. At the same time, estriol can enhance the function of cervix cell, causing the cervix muscle fiber hyperplasia and increasing the cervical elasticity and softness. In addition, estriol has feedback inhibition on the hypothalamus and pituitary but does not inhibit ovulation while only having significant impact on the corpus luteum and therefore can be used as the auxiliary drug in the medium-term labor induction and artificial abortion and for the treatment of various kinds of menopathy. Estriol also has significant effect on the hematopoietic system and can reduce vascular permeability and fragility. Therefore, it can be used for the treatment of various kinds of hemorrhage. It also has effect of rapidly increasing the peripheral leukocytes and generally begins to take effect at 1 to 3 days after treatment but with a shorter duration of action and is effective in treatment the leukopenia induced by radiotherapy and chemotherapy.

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