A woman's decision whether or not to take tamoxifen to help treat breast cancer (or prevent breast cancer) should be made carefully, weighing the benefits and risks to the medication. While the benefits of tamoxifen outweigh the risks for many breast cancer patients, serious complications have been reported in a small percentage of women on tamoxifen. These complications were noted more frequently, and sometimes exclusively, in women over the age of 50. Complications that may occur from the use of tamoxifen include the following:
Endometrial cancer (cancer of the lining of the uterus): Post-menopausal women who take tamoxifen and who have not had a hysterectomy (uterus removal) are at a higher risk of developing endometrial cancer than women who are not on tamoxifen. In the NSABP's Breast Cancer Prevention Trial, endometrial cancer occurred 2.5 times more frequently in women who took tamoxifen compared to women who took a placebo (inactive pill).
However, the risk of endometrial cancer is still low among women on tamoxifen: approximately 2 out of 1000 women on tamoxifen will develop endometrial cancer.
Research suggests that long-term use of tamoxifen among post-menopausal women increases the chances of endometrial problems. According to a review of 106 studies that was published in the journal, Gynecologic Oncology, women who use tamoxifen on a long-term basis are more likely to develop endometrial cancer and endometrial sarcomas than women who have not used tamoxifen. However, the NSABP studies reviewing the incidence of endometrial cancer and tamoxifen use found that earlier, highly treatable endometrial cancers were detected in women on tamoxifen. While women who are diagnosed with early-stage endometrial cancer generally have good prognoses (expected outcomes), the prognosis for advanced endometrial cancer can be far worse.
Because of these findings, women on tamoxifen are encouraged to have yearly gynecological exams. Any abnormal bleeding or uterine pain should be reported to a physician immediately. Women considering tamoxifen are encouraged to have an endometrial sampling (removal of cells in the lining of the uterus for microscopic examination) to make sure there are no pre-existing uterine problems. Researchers are investigating whether women on tamoxifen should receive annual vaginal ultrasound exams to screen for endometrial thickening. However, early studies do not indicate a need for routine ultrasounds and/or endometrial biopsies unless there are specific symptoms such as vaginal bleeding or spotting.
Blood clotting (thrombosis): Women who take tamoxifen have an increased risk of blood clots compared with women who do not take tamoxifen. Blood clots will most commonly form in large veins in the legs (deep vein thrombosis). Sometimes, these blood clots can dislodge and travel to the lungs (called pulmonary embolism). Pulmonary embolism is a serious, possibly fatal condition in which a blood clot causes blockage of an artery in the lungs.
Uterine abnormalities: Some researchers have suggested that tamoxifen can increase a woman's risk of developing uterine tumors. Tamoxifen may make endometriosis worse in women with a history of the condition. Endometriosis is a non-cancerous condition in which endometrial (lining of the uterus) tissue is found outside the uterus in other areas of the body (such as ovaries or the fallopian tubes). Tamoxifen, which can stimulate thickening and growth of the endometrial (uterine) lining, can also stimulate this tissue when it is found outside the uterus, and may increase symptoms associated with endometriosis (such as pain before and during menstruation, pain during or after sexual intercourse, and heavy or irregular bleeding).
In some women, tamoxifen may stimulate the growth of uterine fibroids (bundles of tissue that grow on the wall of the uterus) or other abnormalities on the uterine wall. These conditions may make it more difficult for physicians to diagnose endometriosis.
Cataracts or other eye problems: Though rare, tamoxifen has been shown to cause cataracts (loss of clearness in the lens of the eye) or minor damage in the cornea and retina. Approximately 25 of every 1000 women on tamoxifen developed cataracts in the NSABP's Breast Cancer Prevention Trial compared to approximately 22 of every 1000 women who took a placebo (inactive pill) Women who take tamoxifen should receive annual eye exams and report any eye problems to their physicians.
Stroke: Strokes occur more frequently among women who take tamoxifen than women who do not take tamoxifen. A stroke is a serious condition in which the supply of blood and oxygen is temporarily cut off to part of the brain. Strokes are more common in tamoxifen users who have other risk factors for stroke (such as obesity, high blood pressure, sedentary lifestyle, etc.). The risk of stroke while on tamoxifen may be related to the risk of blood clots (see the section on blood clotting above). Click here to learn more about stroke.
The following chart shows the incidence of side effects in women during the National Adjuvant Breast and Bowel Project's (NSABP) Breast Cancer Prevention Trial (1992-1997):
| Condition | Incidence in women taking tamoxifen | Incidence in women not taking tamoxifen |
| Endometrial cancer | 2.29/1000 cases annually | 0.92/1000 cases annually |
| Pulmonary embolism | 0.75/1000 cases annually | 0.25/1000 cases annually |
| Deep-vein thrombosis | 1.26/1000 cases annually | 0.79/1000 cases annually |
| Stroke | 1.43/1000 cases annually | 1.00/1000 cases annually |
| Cataract | 25.41/1000 cases annually | 22.51/1000 cases annually |
| Cataract surgery | 46.62/1000 cases annually | 31.43/1000 cases annually |
Source: Fisher B, Costantino JP, Wickerham DL, et al. Tamoxifen for prevention of breast cancer: report of the National Surgical Adjuvant Breast and Bowel Project P-1 study. J Natl Cancer Inst. 1998;90:1371-1388. Additional information, National Cancer Institute.
- The National Adjuvant Breast and Bowel Project (NSABP) provides statistics on the side effects of tamoxifen, as noted in their first Breast Cancer Prevention Trial, the B-24 Trial, and other trials at http://www.nsabp.pitt.edu/
- The medical study, "Tamoxifen for Prevention of Breast Cancer: Report of the National Surgical Adjuvant Breast and Bowel Project P-1 Study," is published in the September 16, 1998 issue of the Journal of the National Cancer Institute. An abstract of the study is available at http://jnci.oupjournals.org/
- The American Cancer Society provides information on tamoxifen at http://www.cancer.org/
- The medical study, "Risk and Prognosis of Endometrial Cancer After Tamoxifen for Breast Cancer," is published in the September 9, 2000 issue of The Lancet. The medical study is available online (for registered users) at http://www.thelancet.com/
- The medical study, "Sonographic, Hysteroscopic, and Histologic Evaluation of the Endometrium in Postmenopausal Women with Breast Cancer Receiving Tamoxifen," is published in the February 2000 issue of the Journal of the American Association of Gynecologic Laparoscopists. An abstract of the study is available at http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?cmd=
Retrieve&db=PubMed&list_uids=10648743&dopt=Abstract - The review, "Tamoxifen (ICI46,474) as a Targeted Therapy to Treat and Prevent Breast Cancer," is published in a 2006 issue of the British Journal of Pharmacology (2006 Jan;147 Suppl 1:S269-76).
- The August 2004 medical study, Endometrial Pathologies Associated with Postmenopausal Tamoxifen Treatment, is published in Gynecologic Oncology (2004 Aug;94(2):256-66).
- The medical study, Endometrial thickness of postmenopausal breast cancer patients treatedwith tamoxifen" is published in a 2005 issue of Gynecological Endocrinology (2005 Dec;21(6):312-6).
- The September 14, 2000 Imaginis.com report, "Study Finds Tamoxifen May Significantly Increase Risk of Uterine Cancer," is available at http://www.imaginis.com/breasthealth/news/news9.14.00.asp
- For general information on tamoxifen, please visit http://www.imaginis.com/breasthealth/tamoxifen.asp
