Soma is at once a plant, the juice pressed from that plant, and a god, three things the Vedic tradition does not finally separate. The whole ninth book of the Rigveda, more than a hundred hymns, is addressed to Soma Pavamana, Soma in the act of being purified, sung as the pressed juice is strained through a woollen filter into wooden vats during the sacrifice. The drink was central to Vedic ritual: it was pressed from a plant (whose exact botanical identity has been debated for centuries and remains uncertain), mixed with water and milk, and consumed by the priests and offered to the gods, above all to Indra, who is said to drink it in vast quantities before his great deeds. The hymns describe Soma as producing exhilaration, clarity, inspiration and a sense of expanded power and even immortality.
The plant's botanical identity has been debated since the nineteenth century and remains unresolved. The most influential modern proposal, made by the ethnomycologist R. Gordon Wasson in 1968, identified Soma with the fly-agaric mushroom Amanita muscaria, drawing on Siberian shamanic use of the same fungus; this was challenged by the Indologist John Brough and others who argued for Ephedra, whose stimulant alkaloids suit the martial energy the hymns associate with Soma-drinking Indra better than a mushroom's disorienting effects, and further candidates proposed over the decades include Peganum harmala, Syrian rue, and various Sarcostemma species, with no consensus reached. The debate is sharpened by Soma's close cognate in the Avesta, Haoma, the parallel sacred drink of Zoroastrianism, which points to a shared, and equally disputed, Indo-Iranian original.