What Tamoxifen is

Tamoxifen is a selective oestrogen receptor modulator, and selective is the operative word: it antagonises the oestrogen receptor in some tissues while acting as an agonist in others. That mixed behaviour is not a side effect, it is the defining property of the SERM class and the reason tamoxifen and raloxifene, both SERMs, behave differently from each other. Tamoxifen itself is a prodrug, converted by liver enzymes into the metabolites that do most of the receptor binding, so activity depends on that conversion rather than on the parent compound. It has one of the longest continuous clinical records of any compound in this category.

How Tamoxifen compares

Against clomifene: both SERMs, differing in tissue selectivity and in how strongly each acts at the hypothalamus. Against the aromatase inhibitors: different mechanism entirely, receptor blockade rather than reduced synthesis. Against raloxifene: also a SERM, but with a different tissue profile again, which is why the class is described as selective rather than uniform.