Modern scholars believe that certain examples of berserker rage to have been induced coluntarily by the consumption of drugs such as the hallucinogenic mushroom *Amanita muscaria* (Howard D. Fabing. "On Going Berserk: A Neurochemical Inquiry." Scientific Monthly. 83 [Nov. 1956] p. 232), or massive quantities of alcohol (Robert Wernick. The Vikings. Alexandria VA: Time-Life Books. 1979. p. 285).

This hypothesis was first suggested by the Swedish professor Samual Ödman in 1784. Ödman based his theory on reports about the use of fly-agaric among Siberian shamans. The notion has become widespread since the 19th century, but no contemporary sources mention this use or anything similar in their description of berserkers. Today, it is generally considered untrue or at best speculation that cannot be proven.

Botanists have suggested the behavior might be tied to ingestion of bog myrtle (Myrica gale syn: Gale palustris), a plant that was one of the main spices in alcoholic beverages in Scandinavia. The drawback is that it increases the hangover headache afterwards. Drinking alcoholic beverages spiced with bog myrtle the night before going to battle might have resulted in unusually aggressive behavior.

A UK television programme in 2004 tested the possible use of fly agaric and alcohol by training a healthy volunteer in the use of Viking weapons, then evaluating his performance under the influence of fly agaric or alcohol compared to no influence. It was shown that use of fly agaric or alcohol severely reduced his fighting ability, and the tentative conclusion drawn was that berserk state was achieved psychologically; otherwise, berserkers would have been too easy to kill. On the other hand, the Zulu impi are said to have made use of snuff containing cannabis and/or mushroom-derived psychoactives to enhance their performance in battle.

A simple theory attributes the behavior to drunken rage. It is also possible that berserkers worked themselves into their frenzy through purely psychological processes, perhaps using frenzied rituals and dances. According to Saxo Grammaticus they also drank bear or wolf blood.

While such practices would fit in with ritual usages, other explanations for the berserker's madness have been put forward, including self-induced hysteria, epilepsy, mental illness or genetic flaws (Peter G. Foote and David m. Wilson. the Viking Achievement. London: Sidgewick & Jackson. 1970. p. 285).

U.S. professor Jesse L. Byock claims (in Scientific American, 1995) that berserker rage could have been a symptom of Paget's disease. Uncontrolled skull bone growth could have caused painful pressure in the head. He mentions the unattractive and large head of Egill Skallagrímsson in Egilssaga.

Other possibilities are mild epilepsy, rabies, and hysteria. Nevertheless, these theories are highly unlikely, as the berserkers would - seemingly - inevitably turn against each other instead of the enemies. During battle, they are consistently described in the frenzy of rage; yet, the berserkers, while sometimes purportedly felling allies, seem to have avoided attacking each other.