Hi John,
It's difficult to give any hard specifics without knowing more about what entheogen was being used, the dosing, and what the circumstances were.
For example, early experiments with psychedelics like LSD had very high cure rates for alcoholism, but they found that was only true where there was a "guide" that helped people through the experience and helped them make positive decisions/commitments, etc. (Ken's background might give him some insights into this as well). Something similar goes on with the modern use of Ayahuasca--it seems to be very dependent (especially in terms of positive vs. negative results) on the presence and mediating influence of a good "guide" or shaman.
There is also the question of what is being taken, and perhaps whether a mix is being taken. There are reports of very positive results from a mix of mescaline and meth (added as an energizer). (
https://en.wikipedia.org/wiki/Psychedelics_in_problem-solving_experiment#Results). So, there may have been some synergistic combination at work as well. However, we are very hampered by the fact that we don't know what entheogen(s) was being used and what the active ingredients were.
Likewise, there is the question of dosing. "Microdosing" is a hot topic right now, and usually involves taking very small doses of LSD. The doses do not bring on a psychedelic experience, but are reputed to strongly assist in creativity, concentration, mood, etc. This has been a big deal in Silicon valley among the computer types.
So, questions: (1) Was the plant eaten as part of a ceremony of some type with a "medicine man"/shaman perhaps acting as a guide? (2) What type of dosing (i.e., how much) was eaten? (3) Was it processed or mixed together with other things or plants in some way? (4) Can you describe the plant or plants being used?
It would help if you looked up and studied pictures of the various entheogens of Europe. (I would search combinations of Entheogen and Europe and go through what comes up. Here is one that I saw:
https://www.newworldencyclopedia.org/entry/Entheogen#Europe). There are many mushrooms used for this purpose, but I think you would have recognized a mushroom as being a mushroom rather than a plant. I came across Syrian Rue, which is largely toxic, but has ingredients in its seeds that make it an Ayahuasca analog:
https://en.wikipedia.org/wiki/Peganum_harmala#Recreational_drug_use It is common in the Middle East and North Africa (which would have included areas the tribes traveled through), but I don't know that it is found in Europe.
Anyhow, this is a big question. You would need to do more research in terms of searching the internet and looking at pictures of different plants for us to really hone in on this topic any further.
Cordially,
S&S