Tamoxifen therapy is reported to increase the risk of deep venous thrombosis and pulmonary embolism (DVT/PE). To the authors' knowledge, it is not yet known whether the risk changes with the amount of time elapsed since the initial tamoxifen prescription. This information would be valuable in identifying patients at high risk for DVT/PE.

The relation between timing of tamoxifen use and venous thromboembolism risk was examined. The study population of 16,289 women was identified from the clinical database of the Danish Breast Cancer Cooperative Group. It included women diagnosed with International Union Against Cancer (UICC) stage I or stage II estrogen receptor-positive breast cancer between 1990 and 2004 at ages 45 to 69 years. Risks, risk ratios (RRs), and crude and adjusted hazards ratios were calculated for each of the first 5 years after breast cancer surgery and then cumulatively over the next 5 years.

The 5-year risk of DVT/PE was 1.2% for women receiving tamoxifen and 0.50% for women not receiving tamoxifen. Women treated with tamoxifen were at a higher risk for DVT/PE during the first 2 years after exposure (RR, 3.5; 95% confidence interval [95% CI], 2.1-6.0). Subsequently, their risk was not found to be substantially increased (RR, 1.5; 95% CI, 0.88-2.5). Older women taking tamoxifen appeared to be at higher risk than younger women during the first 2 years of exposure.

The findings of the current study suggest that the first 2 years after the initiation of tamoxifen therapy may be the most crucial time for monitoring DVT/PE risk, particularly in older women.