[1] E1S is most commonly taken by mouth, but in the form of Premarin can also be taken by parenteral routes such as transdermal, vaginal, and injection.

[1][2] E1S itself is essentially biologically inactive, with less than 1% of the relative binding affinity of estradiol for the estrogen receptors (ERs), ERα and ERβ.

[7] The compound acts as a prodrug of estrone and more importantly of estradiol, the latter of which is a potent agonist of the ERs.

[6] Simultaneously, estrogen sulfotransferases transform estrone back into E1S, which results in an equilibrium between the two steroids in various tissues.

[14] This in part led to the introduction of conjugated estrogens (Premarin), which are primarily estrone sulfate, in 1941.