Lots of medicine is not just historically derived from plants but actually manufactured from them - extracting and purifying an active ingredient from plant mas

Lots of medicine is not just historically derived from plants but actually manufactured from them - extracting and purifying an active ingredient from plant mass often is cheaper than synthesizing it chemically, so lots of widely used products from 'big pharma' effectively are herb-based. However, this 'extracting and purifying' step is one that must be done, you don't want to give the patient the whole 'cocktail' of active substances that happen to be in the herb along with the one thing that does have the desireable effect; so skipping that step and using the herb directly often is not appropriate; sure, it can work and have some beneficial effect as it was used historically, but that's something you stop doing once you have the practical ability to do it better by properly extracting the appropriate component.