There is reason to be careful in testing medicines - remember Thalidomide and Debendox?

However the "natural" movement may have a point since apparently one of the naturally occuring components (was it DBD?) in cannabis is an antipsychotic while THC (the yield of which is escalated in stronger cultivated cannabis plants) may be implicated in certain mental problems.

Conversely, things like St John's Wort are thought to have dangers.