SERM is a selective estrogen receptor modulator, which is also a compound that binds with estrogen receptors and exhibit actions in some tissues and anti-estrogen actions in other tissues.
The best SERM would be able to deliver all the benefits of estrogen without the adverse effects. Remember that clomid and nolvadex are both anti-estrogens, belong to the same compound group, and both are structurally related and specifically classified as selective estrogen receptor modulators with mixed agonistic and antagonistic properties. In certain tissues SERM can block the effects of estrogen by altering the binding capacity of the receptor, while in others they can act as actual estrogens that activate receptors.
Both of these drugs counteract the side effects of having too much estrogen on the hypothalamus in men and also heighten the release of gonadotropin-releasing hormones. Though they appear to be related, they have some differences in action. Nolvadex is more anti-estrogenic in the hypothalamus and pituitary. On the other hand, clomid seems to weaken estrogenic activity at the pituitary.