Its interesting that "soma" comes up not only in ancient civilizations, but also in "Brave New World"
and we now have a drug on the market called soma.
(Stop it, Bob...you're leaping again.)
Soma, or haoma (in the ancient Avestan or "Old Bactrian"), probably was a specific plant, perhaps one that was over-harvested and now extinct. Zarathrustra was very concerned about the "injudicious use" of haoma for psychotropic effect rather than it's purely ritualistic use. The word "drug" actually comes from the word "druj" which is an ancient Iranian word meaning "falsehood" or "lie".