There was an actual study published in a medical journal a few years ago which attempted to account for all the minoxidil that gets applied to your scalp when you use Rogaine, including the part that's physically absorbed by your scalp, the part that might come off on pillow-cases when you're sleeping, the amount excreted in urine and feces, etc.
They had a big problem with one of their test-subjects, whose recovered minoxidil was a LOT less than with any of the others! After questioning him, they found out that he had used a hair dryer after applying the minoxidil, to hasten the drying. The hot air had apparently blown a substantial portion of the minoxidil right off his scalp! :woot: