At least three serious candidates have been argued at length and none is established. Wasson proposed Amanita muscaria in 1968; Flattery and Schwartz proposed Peganum harmala in 1989; ephedra is favoured on the grounds that it is the plant used in the living Zoroastrian Yasna, a ritual with no recorded break. Each fits some of the textual evidence and fails on other parts of it.
What would resolve this A securely dated archaeobotanical find from an Indo-Iranian ritual context with residue analysis, or a philological argument that settles the disputed Rig Veda passages. A fourth possibility deserves testing: that the hymns are liturgical praise rather than pharmacological description, in which case a mild plant is sufficient and the search for a potent one is misconceived.
OpenIndology, Iranian and Avestan philology, archaeobotany, ethnobotanyR. Gordon Wasson, Soma: Divine Mushroom of Immortality