A large study sponsored by the French government found that women taking oral estrogens (like Premarin or Prempro) were four times more likely to have a blood clot than women using estrogen in a transdermal gel or patch delivery.  In fact, the women using estradiol gel or patches had no more blood clots than women using no hormones at all (placebo group).  Used successfully by 70% of postmenopausal women in Europe for 30 years, American women now have EstroGel, Estrasorb, Climara, and Vivelle DOT to choose from for bioidentical transdermal hormone replacement.   Currently, only 3% of American women use a transdermal form of estradiol, though studies since the 1970’s have shown better general results and reduced risk of blood clots.  To understand how that happened, read Barbara Seaman’s book “The Greatest Experiment Ever Performed on Women”.