According to a report from the Agency for Healthcare Research and Quality, three drugs that are used to reduce a woman's chance of getting breast cancer have also been shown to cause serious side effects and also carry risks of other adverse effects.

Breast cancer is the second most commonly diagnosed cancer among women (after skin cancer), with more than 190,000 new cases diagnosed each year in the United States and breast cancer is estimated to cause more than 40,000 deaths per year. The National Cancer Institute estimates that nearly 15 percent of women born today will develop breast cancer in their lifetimes. Most cases of breast cancer occur in women with no specific risk factors other than age and gender, although family history of ovarian and breast cancer is associated with higher risk.

The report is based on a review of studies of the use of medications to reduce the risks of getting breast cancer in women.

Tamoxifen, raloxifene and tibolone can be prescribed to women with a family history of breast cancer or other risk factors, but prescribing practices vary widely. According to the study, all three drugs significantly reduce invasive breast cancer in midlife and older women, but benefits and adverse effects can vary depending on the drug and the patient.

The study found that all three drugs do reduce the occurrence of breast cancer but have various side effects. The most common side effects for tamoxifen are flushing and other vasomotor symptoms (e.g., night sweats, hot flashes), vaginal discharge and other vaginal symptoms such as itching or dryness; for raloxifene, side effects include vasomotor symptoms and leg cramps; and for tibolone, side effects include vaginal bleeding.

The study also found that each drug carried risks of adverse effects. Tamoxifen increases risks of endometrial cancer, hysterectomies and cataracts. Tamoxifen and raloxifene increase risk of blood clots, although tamoxifen's risk is greater. Tibolone carries an increased risk of stroke.
References:
1. Heidi D. Nelson, et al. Systematic Review: Comparative Effectiveness of Medications to Reduce Risk for Primary Breast Cancer. Annals of Internal Medicine. 17 November 2009, Volume 151 Issue 10.