If you take progesterone orally, the liver will intercept about 80% and turn the progesterone into progesterone metabolites, an undesirable consequence. Progesterone, applied to the skin, will not be seen by the liver, yielding 98-100% delivery to progesterone receptors (this is where progesterone accomplishes many good benefits to your health).  It is important, therefore, to give attention to best delivery systems when addressing progesterone deficiency.