According to the American Cancer Society, common side effects of estrogen blockers include fatigue, hot flashes, vaginal dryness, vaginal discharge and mood swings. Rarely, more serious side effects such as an increased risk of endometrial cancer or heart attack and uterine sarcoma are possible, as well as blood clots, usually in the legs, that may break off to cause a pulmonary embolism.
In postmenopausal women, another possible side effect associated with the use of estrogen blockers is stroke, notes the American Cancer Society. In premenopausal women, estrogen may cause a thinning of the bones, while in postmenopausal women, it can strengthen the bones. Women with bone metastasis may experience high calcium levels in the blood or pain and swelling in the bones or muscles known as a tumor flare.
Estrogen promotes the types of cancers that are estrogen receptor-positive; two out of three breast cancers are this type, explains the American Cancer Society. This makes taking estrogen-blocking therapies worth the possibility of any side effects. They can be used on their own, in conjunction with other therapies, as a follow-up therapy after cancer treatment ends or if cancer comes back or spreads. However, they do not help cancers that are estrogen receptor-negative.