Researchers have identified a gene variant that significantly increases the risk of blood clots among breast cancer patients undergoing treatment with the drug tamoxifen.
A US study of 412 women who were administered tamoxifen after breast cancer surgery found that those who developed blood clots were five time more likely to carry the Factor V Leiden (FVL) gene.
The mutation is inherited and is more prevalent among people with northern European and Scandinavian ancestry. It is present in around 5% of the UK population.
Tamoxifen is already known to double the risk of blood clots, or thromboembolisms, in women overall, but in people with the FVL gene its impact is even bigger, the scientists said.
FVL facilitates clotting of blood and heightens the risk of deep vein thrombosis and pulmonary embolism - a potentially lethal clot on the lung.
The researchers, led by Dr Judy Garber from the Dana-Farber Cancer Institute in Boston, wrote in the Journal of the National Cancer Institute: "These data may prove useful to women who must decide between tamoxifen and an effective, essentially non-thrombogenic, alternative adjuvant therapy for breast cancer."