About Soma, generic Soma and pain relief:

Soma (or Haoma (Avestan), from Proto-Indo-Iranian *sauma-, was a ritual drink of importance among the early Indo-Iranians, and the later Vedic and greater Persian cultures. It is frequently mentioned in the Rigveda, which contains many hymns praising its energizing qualities. In the Avesta, Haoma has an entire Yasht dedicated to it.
It is described as prepared by pressing juice from the stalks of a certain mountain plant, which has been variously hypothesized to be water, human semen, , alcohol, mead, a psychedelic mushroom, cannabis, Peganum harmala, pomegranate, or ephedra. In both Vedic and Zoroastrian tradition, the drink is identified with the plant, and also personified as a divinity, the three forming a religious or mythological unity. It is considered by some to be the symbolic representation of the Supreme Truth. Also, you may know Soma (Carisoprodol) as a a muscle relaxant, used to relieve the pain and stiffness of muscle spasms.

Etymology
Both Soma and the Avestan Haoma are derived from Proto-Indo-Iranian *sauma-. The name of the Scythian tribe Hauma-varga is related to the word, and probably connected with the ritual. The word is derived from an Indo-Iranian root *sav- (Sanskrit sav-) "to press", i.e. *sav-ma- is the drink prepared by pressing the stalks of a plant (cf. es-presso). The root is probably Proto-Indo-European (*sewh-), and also appears in son (from *suhnu-, "pressed out" i.e. "newly born").

Vedic Soma
In the Vedas, Soma is portrayed as sacred and as a god (deva). The god, the drink and the plant probably referred to the same entity, or at least the differentiation was ambiguous. In this aspect, Soma is similar to the Greek ambrosia (cognate to amrita); it is what the gods drink, and what made them deities. Indra and Agni are portrayed as consuming Soma in copious quantities. The consumption of Soma by human beings was probably under the belief that it bestowed divine qualities on them.

In Hinduism
In Hindu art, the god Soma was depicted as a bull or bird, and sometimes as an embryo, but rarely as an adult human. In Hinduism, the god Soma evolved into a lunar deity, and became associated with the underworld. The moon is the cup from which the gods drink Soma, and so Soma became identified with the moon god Chandra. A waxing moon meant Soma was recreating himself, ready to be drunk again. Alternatively, Soma's twenty-seven wives were the star goddesses, the Nakshatras - daughters of the cosmic progenitor Daksha - who told their father that he paid too much attention to just one of them, Rohini. Daksha subsequently cursed Soma to wither and die, but the wives intervened and the death became periodic and temporary, and is symbolized by the waxing and waning of the moon.
The famous ayurvedic scholar Sushruta wrote that the best Soma is found in the upper Indus and Kashmir region.

Avestan Haoma
The continuing of Haoma in Zoroastrianism may be glimpsed from the Avesta (particularly in the H?m Yast, Yasna 9.11), and Avestan language *hauma also survived as middle Persian h?m. The plant Haoma yielded the essential ingredient for the ritual drink, parahaoma.
In the H?m yašt of the Avesta, the Yazata (divine) Haoma appears to Zoroaster "at the time of pressing" (havani ratu) in the form of a beautiful man. Yasna 9.1 and 9.2 exhort him to gather and press Haoma plants. Haoma's epitheta include "the Golden-Green One" (zairi-, Sanskrit hari-), "righteous" (ašavan-), "furthering righteousness" (aša-vazah-), and "of good wisdom" (hu.xratu-, Sanskrit sukratu-).
In Yasna 9.22, Haoma grants "speed and strength to warriors, excellent and righteous sons to those giving birth, spiritual power and knowledge to those who apply themselves to the study of the nasks". As the religion's chief cult divinity he came to be perceived as its divine priest. In Yasna 9.26, Ahura Mazda is said to have invested him with the sacred girdle, and in Yasna 10.89, to have installed Haoma as the "swiftly sacrificing zaotar" (Sanskrit hotar) for himself and the Amesha Spenta. Haoma services were celebrated until the 1960s in a strongly conservative village near Yazd.

Candidates for the Soma plant
There has been much speculation as to the original Proto-Indo-Iranian Sauma plant. It was generally assumed to be hallucinogenic, based on RV 8.48 cited above. But note that this is the only evidence of hallucinogenic properties, in a book full of hymns to Soma. The typical description of Soma is associated with excitation and tapas. Soma is associated with the warrior-god Indra, and appears to have been drunk before battle. For these reasons, there are energizing plants as well as hallucinogenic plants among the candidates that have been suggested, fly agaric (Amanita muscaria) which was widely used as a brew of sorts among Siberian shamans for its hallucinogenic and entheogenic properties. Several texts like the Atharvaveda extol the medicinal properties of Soma and he is regarded as the king of medicinal herbs (and also of the Brahmana class).
Since the late 1700s, when Abraham Hyacinthe Anquetil-Duperron and others made portions of the Avesta available to Western scholars, 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.
In the late 19th century, the highly conservative Zoroastrians of Yazd (Iran) were found to use Ephedra (genus Ephedra), which was locally known as hum or homa and which they exported to the Indian Zoroastrians. (Aitchison, 1888) The plant, as Falk also established, requires a cool and dry climate, i.e. it does not grow in India (which is either too hot or too humid or both) but thrives in central Asia. Later, it was discovered that a number of Iranian languages and Persian dialects have hom or similar terms as the local name for some variant of Ephedra.

There are numerous mountain regions in the northwest Indian subcontinent which have cool and dry conditions where soma plant can grow. In later vedic texts the mention of best soma plant coming from kashmir has been mentioned. This is also supported by the presence of high concentration of vedic Brahmans in Kashmir up to the present day who setteled there in ancient times because of the easy availability of soma plant.

From the late 1960s onwards, several studies attempted to establish soma as a psychoactive substance. A number of proposals were made, included an important one in 1968 by R. Gordon Wasson, an amateur mycologist, who asserted that soma was an inebriant, and suggested fly-agaric mushroom, Amanita muscaria, as the likely candidate. Wasson and his co-author, Wendy Doniger O'Flaherty, drew parallels between Vedic descriptions and reports of Siberian uses of the fly-agaric in shamanic ritual. (Wasson, Robert Gordon (1968). "Soma: Divine Mushroom of Immortality". Ethno-Mycological Studies 1)

In Western culture
In Aldous Huxley's dystopian novel Brave New World, Soma is the popular dream-inducing drug which is employed by the government as a method of control through pleasure and immediate availability. It is ordinary among the culture of the novel for everyone to use it for whatever various practices: sex, relaxation, concentration, confidence. It is seemingly a single-chemical combination of many of today's drugs' effects, giving its patients the full hedonistic spectrum.

Soma is the central theme of the poem The Brewing of the Soma by the American Quaker poet, John Whittier (1807-1892) from which the well-known Christian hymn "Dear Lord and Father of Mankind" is derived. Whittier here portrays the drinking of soma as distracting the mind from the proper worship of God.

In the book Junky, author William S. Burroughs refers to soma as a non-addictive, high-quality form of opium said to exist in ancient India. He hypothesizes that, were such a drug to exist, drug dealers would be quick to seize on the opportunity and cut the drug until it became generic "junk."
Soma has also been frequently referenced in popular culture.