There is no evidence to support this claim. Neil Price, in his survey of viking warfare and religion (The Viking Way, 2002, pp. 205-6), found exactly two possible indications of "mind-expanding" substances: four seeds of cannibus buried with two women in the Oseberg ship burial in Norway in 834, and a few hundred seeds of henbane in the grave of a woman buried at Fyrkat, Denmark, around 980.

Without any evidence linking mushrooms and vikings, where did this claim come from? In an incisive article on Viking-Age religion, Stefanie v. Schnurbein ("Shamanism in the Old Norse Tradition," 2003) points to the linkage between neoshamanism and drug culture in the 1960s and 1970s. In short, as Schnurbein argues, projecting a drug culture on the Viking Age is more about our own present-day deceits and desires than anything in the primary source material.

Regarding other early medieval armies, these have attracted fewer modern fantasies, at least in terms of TV series and feature films. As such, there's been less of an impetus to research the use of mind-altering drugs by, say, Anglo-Saxons or Franks, much less Lombards or Abodrites, or anyone else for that matter. There's no discussion of drugs, for example, in Guy Halsall's Warfare and Society in the Barbarian West, 450-900 (2003). That's as good an indication as any that no one—insofar as we can know—was using drugs on the early medieval battlefield.

There is almost certainly no truth to the claim. The idea originated as a theory behind the trance-like state in which Berserkers fought in battle. However, there is no evidence they ever took psychedelic mushrooms and it’s unlikely psilocybin would induce the kind of fury of the Berserkers.

It’s more likely that Berserkers were seen as a type of ritual warriors and Berserkergang was encouraged as a traditionally male form of seiðr — a type of sorcery practiced in Norse society. Therefore, Berserkergang could be understood as a religious fervor that Norse warriors would enter before battle.