I know quite a few who have used it transdermally (including me) and have experienced no negative sides from the estradiol. It could be that structurally, for whatever reason, the E can't make the trip - or it could just be that I, for one, am not sensitive at all to E or E related sides (which is why I wasn't too worried about trying it in the first place). However, after a fairly hefty dose for a couple weeks, I ruled that it was too wasteful to go the transdermal route - yeah, it worked, but I'm going to go ahead and use the kits to seperate the estradiol from the prop, removing the ester in the process, and end up with a nice big batch of test suspension.