I found in this article a considerable research about the origins of the Soma-Rasa; the mysterious elixir.
The Encyclopedia of Psychoactive Substances
by Richard Rudgley
Little, Brown and Company (1998)
"Most of these suggestions have been summarily rejected for reasons I will not go into here. Scholars had become rather bored with the whole question as it seemed to many of them an unanswerable one. However, the whole debate was rekindled by R. Gordon Wasson during the late 1960s when he proposed a new candidate for soma - the fly-agaric mushroom. The arguments he put forward are complex; suffice it to say that many distinguished orientalists and other scholars accepted his thesis. In the late 1980s another highly plausible candidate was proposed by David Flattery and Martin Schwartz. Unlike Wasson, who had largely concerned himself with the Indian sources, they concentrated on the Iranian evidence. They suggest that Syrian rue (Peganum harmala) was far more likely a candidate since its hallucinogenic effects are well-known in the Indo-Iranian homeland even today. Their arguments are highly persuasive and convincing".
The Soma of History http://huxley.net/soma/index.html