Women taking tamoxifen for early-stage breast cancer who developed blood clots were more likely to carry a gene mutation for clotting than women taking tamoxifen who did not develop blood clots, according to a study published in The Journal of the National Cancer Institute.

Tamoxifen is a widely-used breast cancer treatment after surgery for pre and post menopausal patients with hormone receptor-positive breast cancer. One of the most serious complications of tamoxifen usage is the development of blood clots, or thromboembolic events (TEs). Previous studies have shown that use of tamoxifen increases the risk of TEs in healthy women twofold, and that in women aged 50 and above, it is associated with even greater risk. The genetic mutation Factor V Leiden (FVL) is the most common inherited clotting factor mutation and also causes increased thrombosis risk.

To determine if having the Factor V Leiden genetic mutation increases the risk of thromboembolisms among women taking tamoxifen, Judy E. Garber, M.D. of the Dana-Farber Cancer Institute, and colleagues at the National Cancer Institute's Cancer and Leukemia Group B (CALGB) looked at 412 women who received the drug as adjuvant treatment for stage I, II, or IIIA breast cancer, between January 1999 and April 2005. The women, whose median age was 64, included 141 patients who developed TEs and 271 who did not.

The researchers found that women who had experienced a thromboembolism while taking tamoxifen were nearly five times more likely to carry the FVL mutation compared with the women who did not develop a thromboembolism. This result differed from previous studies, which found no associated risk of TEs or an increased presence of FVLs among women at risk of breast cancer, but who did not actually have the disease.