There has been much speculation as to the original Rigvedic Soma plant (and of the Proto-Indo-Iranian *Sauma which besides Soma is reflected in the Iranian Haoma).
Since the late 18th century, when Anquetil-Duperron and others made portions of the Avesta available to western scholarship, several scholars have sought a representative botanical equivalent of the haoma as described in the texts and as used in living Zoroastrian practice. Most of the proposals concentrated on either linguistic evidence or comparative pharmacology or reflected ritual use. Rarely were all three considered together, which usually resulted in such proposals being quickly rejected.
The Rigveda calls the plant Soma the "Creator of the Gods" and gives Soma precedence above Indra and the other Gods calling Soma "a God for Gods".
Soma was generally assumed to be entheogenic, based on RV 8.48 Note, there are numerous similar references in the Vedas, e.g. RV9.4, RV9.5, RV9.8, RV9.10, RV9.42. Some descriptions of Soma are associated with tapas (heat, excitement, "energy"). Soma is associated with the warrior-god Indra, and appears to have been drunk before battle. For these reasons, there are stimulant (amphetamine-like) plants as well as entheogenic plants among the candidates that have been suggested. Soma is also often associated with Light and Indra is the "Lord of Light" as shown in RV 8.82.25.
For thee, O Lord of Light, are shed these Soma-drops, and grass is strewn. Bring Indra to his worshippers. May Indra give thee skill, and lights of heaven, wealth to his votary. And priests who praise him: laud ye him.