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Among women taking tamoxifen for early-stage breast cancer, an inherited gene mutation (Factor V Leiden) may increase the risk of blood clots. These results, which differ from a previous study that evaluated women taking tamoxifen for breast cancer prevention, were published in the Journal of the National Cancer Institute.[1]

Tamoxifen is a hormonal agent that blocks the effects of estrogen on a cell and reduces the growth-stimulatory processes caused by the hormone. Tamoxifen may be used for the prevention or treatment of breast cancer. One of the possible side effects of tamoxifen is an increased incidence of thromboembolic events (clotting of the blood), or medical complications caused by blood clots.

Factor V Leiden (FVL) is an inherited gene mutation that affects clotting. An estimated 2-5% of non-Hispanic Whites carry the mutation, with lower prevalence in other ethnic groups.

In a prior study published in the Journal of the National Cancer Institute, researchers evaluated two clotting factor mutations, including FVL, in women taking tamoxifen for breast cancer prevention. In that population, the presence of the FVL mutation did not increase the risk of blood clots.[2]

In order to evaluate FVL and clot risk in women taking tamoxifen for breast cancer treatment, researchers evaluated 124 breast cancer patients who developed a clot during tamoxifen treatment and 248 breast cancer patients who did not develop a clot during tamoxifen treatment. All study participants had Stage I-IIIA breast cancer.

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The FVL mutation was more common among women who developed blood clots than among women who did not develop blood clots (18.5% versus 4.8%).

These results suggest that the FVL gene mutation may increase blood clot risk in women with breast cancer being treated with tamoxifen. The researchers note that clinicians may wish to consider testing for FVL prior to prescribing adjuvant (post-surgery) tamoxifen if a positive test would change treatment decisions.


[1] Garber JE, Halabi S, Tolaney SM, et al. Factor V Leiden mutation and thromboembolism risk in women receiving adjuvant tamoxifen for breast cancer. Journal of the National Cancer Institute. 2010;102:

[2] Abramson N, Costantino JP, Garber JE, Berliner N, Wickerham DL, Wolmark N. Effect of Factor V Leiden and Prothrombin G20210-A Mutations on Thromboembolic Risk in the National Surgical Adjuvant Breast and Bowel Project Breast Cancer Prevention Trial. Journal of the National Cancer Institute. 2006;98:904-10.