Theories about what caused berserker behaviour include ingestion of materials with psychoactive properties, psychological processes, and medical conditions. Modern scholars believe certain examples of berserker rage to have been induced voluntarily by the consumption of drugs such as the hallucinogenic mushroom Amanita muscaria, commonly known as the fly agaric or fly Amanita, or massive quantities of alcohol.
From: http://en.wikipedia.org/wiki/Berserker
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