Estrogen replacement therapy (ERT) is a treatment for women that helps replenish low estrogen levels. This therapy involves taking the hormone estrogen on a regular basis.

The body naturally produces less estrogen after menopause, when the ovaries are removed, or as a result of some other health conditions. ERT can help reverse the effects of low estrogen, which may include severe menopausal symptoms, such as hot flashes and insomnia, and long-term problems, such as weak and brittle bones (osteoporosis).

ERT is also called unopposed estrogen therapy because a second hormone (progestin) is not used along with the estrogen. Estrogen used alone can cause cancer of the lining of the uterus (endometrial cancer). For this reason, unless a woman has had her uterus removed, estrogen usually is combined with progestin; this combined therapy is called hormone replacement therapy (HRT) or simply hormone therapy (HT).