Pharmacokinetics of estradiol
Estradiol can be administered in the form of a very long-lasting subcutaneous pellet implant. These implants are replaced once every 6 to 12 months, and can achieve high and very constant circulating levels of estradiol. They are surgically inserted by a trained physician in a medical office or clinic and can be placed into locations including the lower abdomen, lower back, buttocks, or hips. Subcutaneous implants containing 25, 50, and 100 mg estradiol for replacement usually once every 6 months are available as approved pharmaceutical medications. Up to 800 mg estradiol per implantation has been used. Pharmaceutical estradiol implants have been said to be used almost exclusively in the United Kingdom. An estradiol implant has not been approved by the FDA as a pharmaceutical medication in the United States, but hormone implants, including estradiol implants, are available as custom compounded products in this country.
Pharmacokinetics of estradiol
Estradiol has been assessed for use by rectal administration in a number of studies. Uses of estradiol by this route have included treatment of menopausal symptoms in postmenopausal women. Rectal administration of estradiol is described as qualitatively and quantitatively similar to vaginal administration of estradiol. The use of estradiol by the rectal route considerably bypasses the liver and hence the first-pass metabolism that occurs with oral estradiol, similarly to other parenteral routes of estradiol such as vaginal and transdermal administration. Irritation of the intestines does not usually occur with rectal estradiol. The use of estradiol by the rectal route is not well-accepted by all individuals, and due to its inconvenience, it has been said that rectal administration of estradiol has gained no practical clinical importance.