Blood clots are relatively uncommon and are not a problem for most women using HRT.  However, special consideration is given for those who have had a history of such problems.  There is a debate about what kind of clotting history precludes the use of HRT.  Factors that are taken into consideration include:  frequency of episodes of clotting problems, degree of symptomatic treatment required, whether the woman smokes, and risk factors of other diseases.

Most of estrogen’s clotting effects occur after liver transformation.  Because transdermal (patch or cream) estrogen bypasses this transformation, this form probably would be a better choice for women at risk of circulatory problems. The addition of progesterone seems to have little effect on clotting.

Birth control pills have a dose-related increased risk of clotting problems. The new low-dose pills have significantly reduced this risk.  It is probable that low-dose estrogen in HRT would do the same.  It is important to note that smoking greatly increases the risk of clotting problems with any type of oral estrogen therapy.