Pre-menopausal tamoxifen treatment causes hyperoestrogen production and ovarian cyst formation. Two pre-menopausal breast cancer patients who were treated with tamoxifen developed both permanent supraphysiological oestrogen concentration and ovarian cysts. Serum oestrogen decreased to post-menopausal concentrations and ovarian cysts completely resolved during and following simultaneous treatment with tamoxifen and gonadotrophin-releasing hormone agonist (GnRHa). In pre-menopausal breast cancer patients, GnRHa may prevent possible side-effects of tamoxifen, such as ovarian cysts and supraphysiological oestrogen production.
