From the FDA Drug Label
Venous Thromboembolic Events87 (3)140 (5)0.610.47 to 0.80 Deep Venous Thromboembolic Events48 (2)74 (2)0.640.45 to 0.93 Thromboembolic event** 179(3.2)113(4.6)---- Thromboembolic event** 251(2.1)89(3.6)----
The percentage of patients that had a blood clot with tamoxifen adjuvant therapy compared to an aromatase inhibitor (letrozole or anastrozole) is as follows:
- Venous Thromboembolic Events: 5% of patients on tamoxifen had a venous thromboembolic event, compared to 3% of patients on an aromatase inhibitor (anastrozole) 1.
- Deep Venous Thromboembolic Events: 2% of patients on tamoxifen had a deep venous thromboembolic event, compared to 2% of patients on an aromatase inhibitor (anastrozole) 1.
- Thromboembolic events: 4.6% of patients on tamoxifen had a thromboembolic event, compared to 3.2% of patients on letrozole 2.
From the Research
The risk of blood clots (venous thromboembolism) is approximately 2-3% in patients taking tamoxifen as adjuvant therapy, compared to about 1% or less in those taking aromatase inhibitors, with the most recent and highest quality study suggesting a significant reduction in VTE risk with AI use compared to tamoxifen use 3.
Key Points
- Tamoxifen increases clotting risk due to its estrogen-like effects on coagulation factors, while aromatase inhibitors do not significantly increase clotting risk above baseline 4.
- The risk with tamoxifen is highest during the first 1-2 years of treatment and in patients with additional risk factors such as older age, obesity, history of previous clots, immobility, or recent surgery 5.
- Aromatase inhibitors are often preferred for postmenopausal women due to their lower thrombotic risk, although other factors like efficacy, bone health impacts, and side effect profiles must also be considered when selecting adjuvant hormonal therapy 6.
Evidence Summary
- A 2022 study found that tamoxifen increases thrombin generation, while aromatase inhibitors do not appear to impact hemostasis, suggesting a lack of associated VTE risk 4.
- A 2020 study reported that AI users had a 20% lower all-cause mortality compared to tamoxifen users, without increasing the risk of cardiovascular and thromboembolic events 6.
- A 2019 study found that AI use was associated with at least 41% lower VTE risk compared to tamoxifen use, with greater risk reductions seen in women who also underwent adjuvant chemotherapy 3.