I imagine most of the common medicinal drugs which have originated from plants have already been covered in the other articles but there are two aspects to the subject which have always fascinated me.
Firstly, that the most common and arguably the most effective drugs in everyday use are derived from plants. Secondly, that it would seem from recent research that many of the plants involved have evolved in an almost symbiotic relationship with the human species.
To take the first point; It could be said that the most common drug in use, certainly in the West, is aspirin which was directly derived from Willow bark.
One of the most common heart drugs is Digitalis which is also directly derived from a plant; the Foxglove.
The controversial but widely used Valium was firmly based on the Valerian plant.
Lastly but by no means least is of course the poppy. The painkillers and sedatives taken from this plant are legion and infamous. If you ignore the prejudice against opiates by some in the medical establishment because of their obvious potential for abuse, they are still by far the most effective drugs used in palliative medicine.
The list goes on and can one think of a synthetic drug in such common everyday use which isn't based on a naturally occurring substance?
However what I find even more interesting is that many plant drugs seem to have evolved in a way in which they can only be in existence for use by humans. It seems that their evolution has moved in parallel with the evolution of the human species. For example, many of these drugs (particularly the psychotropics) work by connecting on a molecular level with receptors in the nervous system in the same way as a plug fits a socket. The plugs and sockets, however, are infinitely more complex and specific; only a particular drug can possibly connect. Many drugs for example will have no effect at all on other species. This evolution must have gone hand in hand over hundreds of thousands of years. It suggests that the plants evolved for our specific use and vice versa, which suggests a much deeper relationship with our environment than we realise.