[en] After their prescription, medicines are taken and reach the blood circulation more or less quickly depending on their physico-chemical characteristics, on the excipients etc... and then, diffuse to tissues before reaching their target. The interaction of medicines and their target is followed with a cascade of biochemical mechanisms which lead to the tissue response. From this response the therapeutic effect of these medicines is known. So we can understand that the effect of medicines is not constant, it depends on the time run out from their taking. To describe this voyage of medicines in the human body is essential for increasing the efficiency of medicines and developing new ones. The isotopes have contributed and still contribute to this description of medicines development in the human organism.(F.M.)