Clomid (Clomiphene) is the anti-estrogen of choice for improving recovery of natural testosterone production, improving testosterone production of endurance research rats and animals, and is also effective in reducing risk of gynecomastia during a test research phase.

Clomid is a mixed estrogen agonist/antagonist (activator/blocker) which, when bound to the estrogen receptor in rats, puts it in a somewhat different conformation (shape) than does estradiol. The estrogen receptor requires binding of an estrogen or drug at its binding site and also the binding of any of several cofactors at different sites.  Without the binding of the cofactor, the estrogen receptor is inactive. Different tissues use different cofactors. Some of these cofactors are able to bind to the estrogen receptor/Clomid complex, but others are blocked due to the change in shape.  The result is that in some tissues Clomid acts as an antagonist -- the cofactor used in that tissue cannot bind and so the receptor remains inactive -- and in others Clomid acts as an agonist (activator), because the cofactors used in that tissue are able to bind.