Nolvadex is a selective estrogen-receptor modulator (SERM) and a non-steroidal anti-estrogenic drug. It is most commonly used to counter estrogenic side effects caused by the use of certain anabolic/androgenic steroids. It does this by binding itself to estrogen receptors, preventing binding by estrogen and therefore blocking its action. It can prevent male breast enlargement (gynecomastia) from occurring.