Hormone replacement therapy (HRT) is synthetic estrogen and progesterone (called progestin) designed to "replace" a woman's depleting hormone levels. Decreased levels of estrogen may cause hot flashes, vaginal dryness, sleep disturbances, or other bothersome side effects. The dryness of tissue surrounding the vagina and urethra may cause pain during sexual intercourse or urination. HRT help to relieve these and other menopausal symptoms. Past research has shown that HRT may also help prevent osteoporosis, heart disease, short-term memory loss, depression and other diseases in post-menopausal women. (However, recent research has found that HRT may not provide these benefits and may pose other risks-see sections below for more information). It is estimated that 40 million women will reach menopause in the next 20 years.

While HRT is highly effective at relieving menopausal symptoms and may provide protection against conditions (such as osteoporosis), there is also some evidence that supports a link between HRT and an increased risk for breast cancer. Current research shows that the risk is relatively small and appears to be related to the duration of treatment and the general breast cancer risk of the patient. However, the National Institutes of Health recently halted a large study on HRT because the combined estrogen/progestin regimen of the therapy was found to cause 8 cases of invasive breast cancer for every 10,000 women on HRT (a 26 percent increase in breast cancer risk compared to women who do not take HRT). Though these numbers can be alarming, most experts believe that short-term use of HRT can be safe among women who are not at high risk of breast cancer (such as those with a strong family history of the disease, etc.). Therefore, patients and physicians should make the decision about HRT together, based on the individual medical situation. See the HRT and breast cancer section below for more information. HRT has also been linked to an increased risk of ovarian cancer (with long-term use) and cancer of the uterine lining (in women do not take progestin with estrogen).