The variations observed in the plasmatic half-life of estrogens vary greatly depending on several factors such as dosage, mode of administration, and the patient. Such variations have not been completely explained. Ethinyl estradiol is not converted into estrone; it is quickly absorbed and its maximum plasmatic level is reached in 1-2 hours. The duration of ethinyl estradiol's plasmatic half-life is 1-7 hours for the diffusion phase, and 6-48 hours for the elimination phase. These variations depend on the different methods of dosage and on individual variations among patients. Conjugated equine estrogens at a dose of 1.25 mg/day double the initial estradiol plasmatic level and increase 4 times that of estrone. Natural estrogens, especially estradiol, administered per os are quickly converted by the intestinal wall into estrone and estrone sulphate. In the vaginal route of administration, estradiol, estrone, and estriol quickly pass through the cervical mucus and into the plasma. The nasal and sublingual modes of administration rapidly convert estradiol into estrone, while the absorption rate through the cutaneous mode is even quicker, and subcutaneous implantation guarantees constant plasmatic levels for about 6 months. All different routes of administration have advantages and disadvantages; it is often advisable, when possible, to let the patient choose a preferred mode of administration.