Tamoxifen (trade name Nolvadex) is a drug in pill form, taken orally, that interferes with the activity of estrogen (female hormone). Tamoxifen has been used to treat both advanced and early stage breast cancer for nearly 20 years.
What are the side effects of Tamoxifen?
The following Important Warning appears on the Medline Plus website which is a service of the U.S. National Library of Medicine and the National Institutes of Health.
Tamoxifen may cause uterine cancer, strokes and blood clots in the lungs. Tell your doctor if you have ever had a blood clot in the lungs or legs, a stroke or a heart attack. If you experience any of the following symptoms call your doctor immediately: abnormal vaginal bleeding, irregular menstrual periods, changes in vaginal discharge or pelvic pain or pressure. Talk to your doctor about the risks of taking tamoxifen to reduce the risk of developing breast cancer. In women who do have breast cancer the benefits of tamoxifen outweigh the risks.
The following appears on the Health Canada website:
There is tremendous interest in the use of tamoxifen for the prevention of breast cancer because of its demonstrated effect in decreasing the risk of breast cancer recurrence. The results of three tamoxifen chemoprevention trials have recently been published. The National Surgical Adjuvant Breast and Bowel Project (NSABBP) (P-1) Study found that tamoxifen lowers the risk of breast cancer by 45% among women considered at high risk for breast cancer. The Royal Marsden Hospital trial did not find any benefit of tamoxifen among women who had a family history of breast cancer. A trial conducted in Italy , which enrolled healthy women between the ages of 35 and 70, did not find any benefit for tamoxifen.
Tamoxifen did cause significant side effects such as blood clots and uterine cancer in all of these trials. Because tamoxifen has only been tested in a select group of women, and because of its potential side effects, tamoxifen is not recommended, at this time, for the prevention of breast cancer in all healthy women. Discussion of the relevant and generalized ability of the NSABBP trial results are underway.