TAMOXIFEN AND RISK OF IDIOPATHIC VENOUS THROMBOEMBOLISM

Aims To evaluate a possible positive association between tamoxifen tre atment and the risk of developing idiopathic venous thromboembolism (V TE) in women with breast cancer in the absence of clinical risk factor s for venous thromboembolism other than breast cancer itself. Methods Using information from the large UK-based General Practice Research Da tabase, 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 Janua ry 1, 1991 and December 31, 1996. In a case-control analysis, we compa red their tamoxifen exposure experience prior to the thromboembolic ev ent with that of a randomly selected group of control women with breas t cancer who were matched to cases on age, year of the breast cancer d iagnosis and calendar time. Results We identified 25 cases of idiopath ic VTE and 172 controls, all of whom had boast cancer, but were otherw ise 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 re lative risk estimate of VTE for current tamoxifen exposure, as compare d 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 Ou r study provides evidence that current use of tamoxifen increases the risk of idiopathic venous thromboembolism.