Both oral contraceptives prescribed for birth control and hormone replacement therapy (HRT) prescribed during the menopause contain the female hormone oestrogen. Oestrogen can cause blood to clot more easily by affecting the balance of the coagulation system, putting patients taking these therapies at increased risk of developing a VTE.
The influence of specific types of combined oral contraceptives on the risk of thrombosis remains one of the most important safety issues for these drugs. It is also known that for women who also have a specific thrombophilia, the risk of VTE is further increased.
For post-menopausal HRT users, studies show a two-fold to four-fold increased risk of VTE and the risk appears to be highest in the first year of use.
Figure: hormone-based treatment, such as the contraceptive pill and HRT, can cause the blood to clot more easily.