Hans’ talk will try to illustrate how a scientific basis of understanding is essential for rational decisions on where to use medicinal plants and where they may not be able to deliver benefits comparable to modern pharmaceutical drugs. It will also include discussion of the importance of dosage (“Solely the dose determines that a thing is not a poison.”, Paracelsus, 1538), purity, and reproducibility. A prime example comprises antimicrobial chemicals from plants; they can kill bacteria and funguses very effectively, which enables their use for antibacterial skin creams. Some plant-derived chemicals have also been touted for anticancer effects, but here the problem often is that they are as toxic to surrounding normal cells as they are to cancer cells. Other human health issues, such as dementia and diabetes, originate from protein biochemical or cellular causes that are very unlikely to be amenable to plant based cures. Understanding the causes of health issues and the biochemical actions of plants-derived chemicals should be used to assess the viability of use of medicinal plants for particular applications.