An estrogen ester is an ester of an estrogen, generally estradiol but also estrone or estriol. Esterification renders these drugs into metabolically-resistant prohormones of their respective parent estrogen, improving oral bioavailability. In addition, with intramuscular injection, they have improved lipophilicity, which results in a longer duration when given as a depot in oil. Estrogen esters are used in hormone replacement therapy and as hormonal contraceptives, among other indications.
The first estrogen ester to be marketed was estradiol benzoate in 1936, which was followed by many more. One of the most widely used estrogen esters is estradiol valerate, which was first introduced in 1954.