For those of you who've ever wondered where the phrase "went berserk" comes from, berserkers were a band of mouth-foamingly fierce (and frequently naked) Norse warriors who struck fear into the hearts of every respectable citizen north of the equator during the Viking Age. 

It’s long been believed that the beserkers used psychedelic mushrooms before battles to enter into a savage, trance-like state that allowed them to feel no fear and go wild in the melee. However, a new study has suggested that this might not be strictly true. 

While these bad boys were undoubtedly high on some kind of mind-altering substances, it turns out they were most likely loaded on a lesser-known substance, henbane (Hyoscyamus niger).

In the Journal of Ethnopharmacology, ethnobotanist Karsten Fatur from the University of Ljubljana in Slovenia explains why henbane is a much more plausible source of the beserker’s psychotic fury, as opposed to other theories such as hallucinogenic mushrooms, drunkenness, mental illness, or psychosis from the fungal disease ergotism.

Amanita muscaria, a red-capped mushroom found across parts of Eurasia where Vikings roamed, is one of the most frequently suggested theories. It’s psychoactive properties lie in the compound ibotenic acid, a potent neurotoxin that’s actually used as a “brain-lesioning agent” during experiments on lab rats and monkeys.

If ingested, it also causes a range of psychoactive effects, such as dizziness, hallucinations, and delirium. It can also lead to hyperthermia and intense sweating, which would explain reports that the berserkers would shed their armor and strip naked before charging into battle. However, it also features a reel of nastier effects that you wouldn’t want to experience during hand-to-hand combat, like trembling, vomiting, and diarrhea.

Perhaps, this study argues, that henbane is a more likely suspect. Although native to the Mediterranean, henbane was likely introduced to Scandinavia during the Roman Iron Age. The study notes that the presence of henbane seeds at a number of Viking-age settlements in Denmark and Finland has suggested that they were widely used during the Middle Ages too. 

Not only is henbane an effective pain killer, but the psychedelic state it produces is often associated with one aspect of the beserkers' notorious fighting style: pure rage. 

“This anger effect can range from agitation to full-blown rage and combativeness depending on the dosage and the individual's mental set,” the authors write. “As this is perhaps the most defining component of the berserker state, this symptom is of central importance in identifying potential causes, and provides a very solid reason as to why H. niger is a more appropriate theoretical intoxicant for the berserkers than A. muscaria."

It was often reported that the beserkers were so enraged and frenzied that they were unable to distinguish between friends and foes in the heat of a battle. Remarkably, an inability to recognize faces is also a known symptom of being intoxicated by henbane-like drugs. 

Furthermore, Fatur argues, it's no coincidence that the berserkers were said to fall into a tired hungover-like comedown after their trance-like states. The aftereffects of henbane do, indeed, including symptoms like headache and blurred vision, while mushrooms don’t typically produce many nasty aftereffects. 

Fatur concludes that though there is evidence that matches what we know, it is still speculative, as there are other aspects of the berserkers' famous behavior – like biting shields and chattering teeth – that are unexplained. Still, they weren't the first, and they won't be the last to wake up naked with a hangover and no idea what happened the day before.