I've always been interested in herbal medicines and took a course some time ago to become a qualified herbal practioner...unfortunately the course promoted an holistic approach...life style diet and so on...which was something I'd agreed with so was not a problem for me, but does cause problems whenever anyone asks for advice.
Suggest a person change what they eat and it falls on deaf ears 'But I've always eaten it' which is probably why they are unwell...taking up a hobby is met with a blank stare and the answer 'I haven't the time', time enough to watch television but not time to learn to knit or a foreign language...and so on. And herbal preparations don't act instantly and they can be addictive and downright dangerous...just because the cough mixture is made from leaves doesn't mean you can drink it like water.
Present day scientists are beginning to really pay heed to the herbal medications of the past...Chinese scientists reproduced a recipe for malaria which was in use 2,000 years ago...using the herb Wormwood. They found it was just as effective as modern pharmaceuticals used in the treatment of malarial fevers.
Unfortunately the amounts to be used are either missing from the recipes of many herbal cures or are intended to be used on a large scale which can be impractical in a laboratory setting. So there is a necessary amount of tweaking and experimenting to be done.
A recipe from a book written by Nicholas of Salerno in the mid 1100's for a requies magna, or great rest, contained Opium , Mandrake root , Henbane and Deadly Nightshade. It was used a sleeping draught, probably after wounds suffered in battle, and has been found to be as effective as present day Morphine which itself is a derivative of Opium.
Many skeletons unearthed during the course of excavations have been found to have suffered old, healed wounds. Sometimes losing a limb or having a severe skull fracture...it does point towards the Early Mediaeval herbalist having had effective salves and ointments if those men recovered from what were traumatic injuries.
It's doubtful that those healers knew of the value of vitamins, but included in the general care of an ill or sickly person was the advice to use fresh cabbage, garlic and Pomegranates in their diet...
Garlic grows wild and was probably as easily obtainable in 1100 as it is now...and we know cabbages were grown as both food for humans and a fodder crop for farm animals...but Pomegranates come from Indonesia. Which brings us neatly back to the Merchants who came over land and sea bringing fruits etc to sell.
And A Word of Warning
Don't be taken in by the description 'Natural' on a jar of a herbal remedy...ask advice before embarking on any course of herbal preparation...and always, always tell the pharmacist, herbalist or Doctor which pharmaceutical medicines you are taking before you take any herbal. Many interact violently with proscribed medication.