Arimidex belongs to a category and class of drugs known as aromatase inhibitors (AIs). Aromatase inhibitors belong to an even broader class of drugs known as anti-estrogens . The other subcategory of drug under the anti-estrogens category is known as selective estrogen receptor modulators (SERMs), such as Nolvadex and Clomid . AIs and SERMs make up anti-estrogens. Aromatase inhibitors differ greatly from SERMs in their action and how they deal with the issues of estrogen control. The misunderstanding that SERMs, such as Nolvadex and Clomid, serve to lower estrogen levels must first be addressed. This is a persistent rumor among the anabolic steroid using community that has begun to erode as of late, but the rumor still persists. SERMs serve to block the action of Estrogen at the receptor sites in breast tissue by occupying the receptor sites in place of Estrogen so that Estrogen itself cannot exert its effects there through receptor site binding. Conversely, SERMs will also act as Estrogens at receptor sites at other cells in other areas of the body (the liver, for example in Nolvadex’s case). SERMs do not lower circulating levels of Estrogen in blood plasma. Aromatase inhibitors serve to do this by eliminating the production of Estrogen through binding to and disabling the aromatase enzyme, which is the enzyme responsible for the conversion (or aromatization) of androgens into Estrogen.