Hormone replacement therapy (HRT) can have several adverse effects. One significant risk associated with HRT, especially if estrogen is given without progesterone to women with an intact uterus, is the increased risk of endometrial cancer. Estrogen stimulates the lining of the uterus, and without the balancing effect of progesterone, this can lead to endometrial hyperplasia and potentially cancer. Other risks include breast cancer, blood clots, and stroke, but endometrial cancer is a specific concern with unopposed estrogen therapy.

Goodman, N. F., et al. (2011). American Association of Clinical Endocrinologists medical guidelines for clinical practice for the diagnosis and treatment of menopause. Endocrine Practice, 17(6), 1-25.