I wrote previously about 'Rigvedic Soma' which I will repost here for you:

So one clear candidate for "bygone drugs" for discussion in this thread would be a plant commonly referred to as "Rigveda [ṛgveda] Soma", based on its attestation in that Hindu Vedic text (although it is mentioned elsewhere too). I say discussion, rather than a clear cut answer though, for a reason, as it is a substance that is less 'no longer in existence' than it is a substance which there has been great debate over what it actually is, with a number of different analogues proposed by various scholars. What we do know is that it had ceremonial use, "specifically intended to induce a state of intoxicated ecstasy that produced spiritual health, renewal, and immortality through mystical communion with deity." It wasn't recreational in purpose here, to be sure, but is broader evidence of both ancient drug use, and use specifically of a drug we're unsure on (there is no reference to recreational enjoyment or misuse in the Vedic texts at least).

Westerners have been trying to figure out just what "Soma" is at since the early 1800s. Early proposals from that period included Ruta graveolens - an herb known as 'common rue'; Sarcostemma brevistigma - a type of milkweed; and Ephedra sinica - 'Chinese ephedra', an herb with some medicinal properties; but a full list goes into the dozens. Sarcostemma appears to have been an especially popular identification in that period, but became more and more rejected by the end of the century, while Ephedra remains one of the more enduring of the earlier proposed candidates with supporters still writing favorably for it in recent times.

Cannabis has been a popular identification, especially favored by the early 20th century author Mukherjee, although the first proposal came in 1884, and there are verses which can be read to support it, such as:

The immortal green-tinted Soma when purified is arranged

And some scholars believe there is at least a compelling argument for it. B.G.L. Sawmy summed up the argument for this identification positively, although less than conclusively:

Thus there is reason to infer that it was the water-soluble factor of the Cannabis narcotic and that too in great dilution which opened up their 'doors of perception' and gave vocal expression to their mystic experiences.

But one of the most common identifications which pops up is that Soma was some sort Amanita muscaria, a psychoactive fungus called 'common fly-agaric', a red-capped mushroom. This identification is based on the work of R. Gordon Wasson, who published his theory in 1968 and then continued to revise it (he claims to have first hit on the possibility while chatting with Aldous Huxley). Is key arguments hinge on the description of "Soma" as a "not refer[ing] to the roots or leaves or blossoms or seeds", and that it was a preparation with "inebriating property although non-alcoholic". Others have pushed back against it though, especially Vedic scholars who believe he did not exercise the necessary caution in taking the Vedic text too literally, such as the description lacking roots and such being a passage intended to allude to the divine origins of the plant rather than a literal description of it.

Many also push back that Wasson decided on fly-agaric and then went fishing for verses to support his theory (Mushrooms were his thing), and even some who agree with his end-theory find his method to be suspect, as at least some of his work seems to have been based off observations of Siberian peoples who used the fungus and then simply extrapolating some of his findings there, such as drinking the urine of a person who had previously consumed the plant. There is no hard evidence for it in the Vedas, and Wasson seems to be grasping at very weak evidence there. In Wasson's defense at least, he did at least seem consider other possibilities beyond fly-agaric, although always with a focus of psychoactives, and his recollections are imperfect. Early correspondence while working on his theory do point to his consideration of Lagochilus inebrians - " inebriating mint" and Stropharia cubensis - gold caps, aka "magic mushrooms"; as well as the seeds of various forms of morning glory, which contain lysergic acid.

Wasson's theory is on the outs though it would seem, as more recent archaeological excavations have uncovered evidence of cannabis, ephedra, and opium poppies in the temple sites where the ceremonies were conducted, dating to the mid-1500s BCE. If or how they were used specifically in the preparation of Soma of course is not clear though, nor how widespread their preparation style was, so it isn't quite the final argument. The evidence for Cannabis use points to it being a 'fumigant', the smoke unleashed in the chamber, so possibly not as an ingredient, or of course possibly just with multiple uses.

One additional thing ought to also be noted however. While the hunt for identification of ṛgveda Soma was about finding what it was originally, few disagree that there was wide variety of "surrogate substitutes" in later eras following migratory transitions by practitioners who found themselves in regions where the original Soma no longer could be found or grow. Even Wasson, who disagrees with the Cannabis or Ephedra identification, would agree that they were substitutes used later on, as well as other possible candidates that were proposed as the "original".

Ruck, Paul & Carl Anton. 2016. "Mushroom sacraments in the cults of early europe". NeuroQuantology 14(1) 68-93

Riedlinger, T.J. 1993. "Wasson's Alternative Candidates for Soma". Journal of Psychoactive Drugs. 25(2):149-156. 1993.

Shah, N.C. 2015. "Soma, an Enigmatic, Mysterious Plant of the Vedic Aryas: An Appraisal". Indian Journal of History of Science 50 (1) 26-41

Swamy, BGL 1974. "Sources for a History of Plant Sciences in India II. The RG Vedic Soma Plant." Indian Journal of History of Science 11(1) 11-32