Letrozole (Femara) 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 before delving into any further details.