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.
Estradiol benzoate
Estradiol benzoate is available in Europe and in other parts of the world. It was previously available for medical use in the United States, but is no longer marketed in this country. However, it is approved and marketed in the United States for veterinary use as a subdermal implant both alone and in combination with the androgen/anabolic steroid trenbolone acetate (brand names Celerin and Synovex, respectively). Outside of the United States, estradiol benzoate is also marketed in combination with progesterone for use as an intramuscular injection.