From the Million Women Study April 2015
Different types of HRT have different effects on risk of blood clots
Women taking hormone replacement therapy (HRT) have an increased risk of blood clots in the veins (this is known as venous thromboembolism, and includes deep vein thrombosis or DVT – formation of blood clots in the deep veins of the legs – and the less common, but more serious, pulmonary embolism, blood clots in the lungs) BUT WHEN TAKEN ORALLY, AND ESPECIALLY WHEN A SYNTHETIC VERSION OF PROGESTIN IS ADDED.
TOPICALS OR VAGINAL APPLICATION DO NOT HAVE THIS PROBLEM.
In the Million women study, a reanalysis looked at the effects of different types of HRT on the risk of serious blood clots causing hospital admission or death (Sweetland, Beral et al 2012).
The authors confirmed that the risk is highest for women taking oral (pill) forms of combined oestrogen-progestagen (synthetic) HRT (about a 2-fold risk, compared with women not taking HRT), somewhat raised for women taking oral oestrogen-only HRT (about a 40% increase in risk);
and NOT RAISED AT ALL for women using HRT gels or patches or taking it vaginally!
This is thought to be because when HRT is absorbed through the skin, it has less effect on the liver, and on chemicals involved in blood clotting, than HRT taken by mouth.