A meta-analysis of observational studies showed that oral estrogen, but not transdermal estrogen, increased the risk of venous thromboembolism.

References and Resources
Canonico M, Plu-Bureau G, Lowe GD, Scarabin PY. Hormone replacement therapy and risk of venous thromboembolism in postmenopausal women: systematic review and meta-analysis. BMJ. 2008 May 20. [Epub ahead of print].

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