Tamoxifen

Tamoxifen is a SERM (selective estrogen receptor modifier). It acts as an estrogen receptor (ER) antagonist in some tissue (in this case, breasts) and an ER agonist in other tissue (in this case, bone and uterus). It is often used for breast cancer treatment for this reason, as it prevents osteoporosis and the spread of estrogen-sensitive cancer in the breasts and can be combined with GnRH blockers (either GnRH antagonists or agonists) for treatment.

In other words, tamoxifen acts as an anti estrogen in the breasts but as estrogen in bones and skin.

This medication would likely be effective in halting breast growth (as it would prevent estrogen from acting in the breasts), and is already used for the treatment of fibrocystic breasts and breast cancer due to lack of side effects compared to other medications like danazol. It is sometimes used in treatment of gynecomastia.

It can possibly lower levels of IGF-1 (insulin growth factor 1), but also can lead to higher levels of estrogen in the uterus, which could lead to uterine cancer (though the risks are small).

Unlike tamoxifen, raloxifene is an ER antagonist in uterine tissue (lowering the risk for uterine cancer), and has a greater ER affinity in bone.

Tamoxifen has been proven to lower fibroglandular and breast volume/density in premenopausal women, and it and other SERMS have been proven to move fat distribution from the butt and central region to the legs. https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3005955/

Some medications may interfere with tamoxifen! They include paroxetine (Paxil), fluoxetine (Prozac), and bupropion (Wellbutrin) which inhibit metabolization of tamoxifen to a form the body can use. (https://www.health.harvard.edu/newsletter_article/antidepressants-and-tamoxifen)

Side effects
Tamoxifen can cause cognitive dysfunction, increase the risk of stroke and pulmonary embolism, , and increase the risk of endometrial cancer and cataracts.