• tamoxifen;
  • idiopathic venous thromboembolism

Aims To evaluate a possible positive association between tamoxifen treatment and the risk of developing idiopathic venous thromboembolism (VTE) in women with breast cancer in the absence of clinical risk factors for venous thromboembolism other than breast cancer itself.

Methods Using information from the large UK-based General Practice Research Database, we identified, within a cohort of more than 10 000 women with breast cancer, all women who developed a first-time diagnosis of deep vein thrombosis or pulmonary embolism of uncertain cause between January 1, 1991 and December 31, 1996. In a case-control analysis, we compared their tamoxifen exposure experience prior to the thromboembolic event with that of a randomly selected group of control women with breast cancer who were matched to cases on age, year of the breast cancer diagnosis and calendar time.

Results We identified 25 cases of idiopathic VTE and 172 controls, all of whom had breast cancer, but were otherwise free from other risk factors for VTE. Past tamoxifen exposure was not materially associated with an elevated risk of developing VTE, and we therefore combined never and past users as reference group. The relative risk estimate of VTE for current tamoxifen exposure, as compared with never and past use combined, was 7.1 (95% CI 1.5–33), adjusted for body mass index, smoking status and hysterectomy status. High body mass index was an independent predictor of VTE itself.

Conclusions Our study provides evidence that current use of tamoxifen increases the risk of idiopathic venous thromboembolism.