Oral HRT (oestrogen in tablet form) can increase the risk of blood clots in blood vessels, such as deep vein thrombosis in the legs and pulmonary embolism in the lungs.
By contrast, oestrogen taken through the skin (transdermal oestrogen) in the form of patches, gels, and sprays does not carry these same risks. Some of the older synthetic types of progestogen can also increase the risk of blood clots by a small amount.
Of course, you may already be at risk of blood clots if you smoke, are overweight or have had a blood clot in the past. If this is the case, transdermal HRT should not add to your risk.
As with the risk of blood clots, HRT tablets increase the risk of stroke. Taking oestrogen through the skin at normal doses does not carry the same risk. The risk of stroke increases as you get older, so for younger, healthy women, the blood clot risk for HRT is low.