Side effects of hormone therapy can include hot flashes and other menopausal symptoms.

Side effects of hormone therapy can include hot flashes and other menopausal symptoms.(AROMASIN/NOVARTIS/HEALTH)

If your breast cancer tumor is estrogen- and/or progesterone- receptor (ER/PR) positive, that's good news because it means you have more treatment options and may benefit from hormone therapy. Tamoxifen—which has been around for decades—has been shown to prevent the chances of a recurrence in high-risk women by almost 50%. And in March 2008, researchers announced that drugs in a newer class called aromatase inhibitors (AIs) stopped breast cancer recurrence for postmenopausal patients who had completed tamoxifen treatment (even years after).

In the vast majority of cases, a woman whose cancer is positive for estrogen is also positive for progesterone. "If you're positive for both, we know you will be much more responsive to anti-estrogen therapy," says Julie R. Gralow, MD, director of breast medical oncology at the University of Washington and Fred Hutchinson Cancer Research Center in Seattle.

Two types of anti-estrogen drugs
Tamoxifen (Nolvadex) and raloxifene (Evista) are part of a drug class called selective estrogen receptor modulators (SERMs). "These go into the cell and bind to the estrogen receptor. They shut down the receptors and stop cell growth and division associated with that receptor," explains Dr. Gralow.

Or you can take an AI: anastrozole (Arimidex), letrozole (Femara), or exemestane (Aromasin). "They work by taking away the last bit of estrogen that's being made," Dr. Gralow says.

Premenopausal women whose cancer is ER/PR-positive will nearly always get a SERM, says Dr. Gralow. But if you're postmenopausal, you may be given an AI.

Side effects
The side effects of tamoxifen and other SERMs can include hot flashes, vaginal discharge, and other menopausal symptoms, such as irregular periods, headaches, and fatigue. SERMs may also raise a woman's risk for uterine cancer, so regular pelvic exams are recommended.

Bone thinning and joint pain are potential side effects of the AIs. SERMs may actually have a beneficial effect on a woman's bones.

Tamoxifen may not be right for older women
You can expect to take any of these drugs once a day for five years—although a 2007 study in the journal Cancer suggests that tamoxifen may be on its way out for breast cancer survivors who've passed menopause: Two multicenter trials found that when women who'd been taking tamoxifen for two or three years switched to an AI, their survival rates significantly improved compared with continuing on with tamoxifen. AIs also have another big bonus: fewer long-term side effects, save the bone-density issue.