I believe it is primarily an anti estrogen, that it was marketed to to be like tamox in Japan, a breast cancer drug.
I used it years ago and had some gains. Stuff seemed drying...
it's based on an obscure Japanese breast cancer drug, epithiostanol. This is basically a 5a-reduced steroid with a sulphur bridge connecting the 2- and 3-carbons.
Epithiostanol (epi- indicating the bridge, thoi, indicating sulphur, stan for the androstane skeleton, and ol for the hydroxyl at 17b)
17a-methylepithiostanol is the scientific name for epistane;
probably does not have direct anti-estrogen properties. It probably has indirect action through its androgenic properties,
part of epi's pharmacological activity is do to its conversion to DMT (desoxymethyltestosterone). How much the unconverted epi contributes who knows.