Tamoxabolic-20 (tamoxifen citrate, nolvadex) is a non-steroidal agent which demonstrates potent antiestrogenic properties. These antiestrogenic effects are due to its ability to compete with estrogen for binding sites in target tissues such as in the breast. Medically, the effects of tamoxifen make it very useful in breast cancer patients.
Male bodybuilders however, find tamoxifen very effective in combating gynocomastia (female breast tissue development). Gynocomastia is caused by excess estrogen in the male body. In the steroid user, this happens when anabolic steroids "aromatize" into estrogens. So with tamoxifen competing for and occupying estrogen receptors, the buildup of tissue is greatly reduced and often avoided.
It should be noted that tamoxifen also seems to slightly reduce to gains made during a steroid cycle. Some believe estrogen plays a role in keeping androgen receptors open. This may explain why tamoxifen would slightly hinder gains. Regardless of this effect tamoxifen is one of the most popular anti-estrogen used by athletes. One should remember though that many athletes find no need for tamoxifen while others absolutely need it to avoid a gyno problem.