Traditional use of medicinal agents: a valid source of evidence.

Traditional knowledge uncovered by serious historical research may help to identify new therapeutic agents. It also plays a part in safety evaluation and drug regulation. So far, however, this way of gaining knowledge is very much neglected in the concept of evidence based medicine, which is in no way contradictory to traditional use. Thus, the true meaning of the word 'tradition' is described, while reliable historical sources are quoted. It is then shown how research into the traditional use of medicinal plants is able to reveal knowledge about efficacy and safety of natural products. Several examples successfully leading to new therapeutic options are given. It is then referred to the concept of 'social validation' developed by the Canadian medical historian John Crellin, who also established guidelines for respective research. Eventually, it is proposed to modify the list of evidence levels by inserting an evidence level 4b 'Traditional use in more than one regional cluster'.