Herodotus claimed that the Scythians would smoke marijuana by sitting in a tent filled with smoke. Is there any truth to this claim?

As someone quoted below, this account is found in Histories 4.73, 75:

After the burial the Scythians cleanse themselves as follows: they anoint and wash their heads and, for their bodies, set up three poles leaning together to a point and cover these over with wool mats (περὶ ταῦτα πίλους εἰρινέους περιτείνουσι); then, in the space so enclosed to the best of their ability, they make a pit in the center beneath the poles and the mats and throw red-hot stones into it. . . . the Scythians then take the seed of this κάνναβις (kannabis) and, crawling into the tents, throw it on the red-hot stones, where it smoulders and sends forth such fumes that no Greek vapor-bath (πυρία) could surpass it. The Scythians howl in their joy at the vapor-bath

It's natural to think that this description is based on something real. For one -- although the πυρία (vapor-bath) here was familiar to Herodotus -- this appears to be the first occurrence of the word "cannabis" that we have (the oft-cited Sumerian kunibu is a red herring).

In terms of actual archaeological evidence, this passage can be found in Zohary, Hopf and Weiss (2012), specifically in reference to burial practices:

Such a practice is attested by a special find in a series of fifth-century BC frozen kurgans (burial mounds) at Pazyryk, Siberia. A 1.2 m-high wooden frame was found within each burial, and a bronze vessel inside it. Stones and hemp seeds were found inside each vessel. 'A leather pouch with hemp seeds provided supplies, and scattered hemp, coriander and melitot seeds were also recovered' (Rudenko 1970, as cited by Merlion 2003; Sherratt 1995). Despite the appeal of such a find, hemp seeds can be confused with those of Panicum miliaceum (Bakels 2003), for example, re-identification is therefore advised. At contemporaneous sites in Ukraine, Pashkevich (1999) found Cannabis seeds in several Scythian sites.

Also, note again that Herodotus only mentions the seed (σπέρμα) of cannabis plants being used. There's a conspicuous absence of a reference to flowering material (which also isn't mentioned in archaeological material, though /u/PapaFranz astutely notes that "just because only seeds have been found doesn't necessarily indicate only seeds were being burned"), which would really be the intoxicating agent.

In conjunction with all this: it's interesting to read the accounts of the earliest explorers/conquistadors of the Americas, re: Native American tobacco use. Although I'm unfamiliar with their methods of ingestion and its effects, they have the not-so-subtle ring of great exaggeration. [Edit: alternatively, if not exaggeration, it may be that the "intoxication" here has just as much to do with natural, "placebo"-induced shamanic trance as it does the tobacco itself -- as I've mentioned in the comment below.]

Addendum: Benet (in Rubin 1975) argues -- somewhat tenuously, IMO -- that several modern traditions in E. Europe suggest that "[h]emp never lost its connection with the cult of the dead": mentioning "the throwing of a handful of seeds into the fire as an offering to the dead during the harvesting of hemp," as well as that

[e]ven today, in Poland and Lithuania, and in former times also in Russia, on Christmas Eve when it is believed that the dead visit their familiar, a soup made of hemp seeds, called semieniatka, is served for the dead souls to favor. In Latvia and the Ukraine, a dish made of hemp was prepared for Three Kings Day.