What medicine. used as a breast cancer prevention after surgery?

Depends on menopause: If you are asking about the medications that are used after surgery for early breast cancers to prevent recurrence, the choice depends on whether periods have stopped or not. Tamoxifen is often used if a woman is still menstruating. It blocks the effect of the estrogen she is making. If she is past menopause, "aromatase inhibitors" block the production of estrogen made by her own fat cells.

Depends: Medicine used to decrease the chance of breast cancer returning after "curative" surgery is chosen based upon the characteristics of the cancer. Possiblities include anti estrogens(tamoxifen) aromatase inhibitors, chemotherapy, biologics-e.g.Herceptin (trastuzumab) or combinations. Talk to your oncologist for explanation and recommendations.