The term "transdermal" means that a medication can go through the skin and into the bloodstream to have effects. Most commonly this is in a patch form, and special ingredients must be present to help the medication cross through the skin’s natural barriers. Some other examples would be blood pressure or nicotine patches, however in most cases the absorption through the skin can vary a lot. Other topical THC products are not designed to penetrate all the way into the bloodstream, just to the local nerve endings, so if THC doesn’t reach the bloodstream, it doesn’t have psychotropic effects. I hope you find a product that works well for you!