There are various theories on how their battle frenzy was induced, but amongst them is the idea that some drug or narcotic was consumed. Leading the candidates these days is Amanita muscaria, or fly agaric, the red mushroom with white spots so familiar to fairy tales (Howard D. Fabing. "On Going Berserk: A Neurochemical Inquiry." Scientific Monthly. 83 [Nov. 1956] p. 232).
So how did the berserkers administer the fly agaric? Experimental archaeology offers an alternative to academic theory.
Eating 1-3 mushrooms, shaman-styleEating the mushroom in large doses (1-3 mushrooms), a method still in use amongst indigenous peoples in remote areas of Siberia and other northern regions, induces a shamanic trance rather than a battle frenzy. The ibotenic acid and muscimol overpower the ingester, who becomes literally "away with the fairies".
A bite of mushroomEating a smaller dose, for example a bite-sized piece of mushroom, does give an energy boost and a clearer mind. In fact, a folk name of the Amanita muscaria is "raven's bread", suggesting that Odin's ravens, Huginn and Muninn (literally "thought" or "cognition" and "memory"), were partial to a raven-sized bite. A human taking a bite-sized piece receives noticeably increased stamina and imperviousness to cold. The berserkers were known to travel long distances through the snow. In these amounts, the ibotenic acid and muscimol are not as apparent, and it is rather the effects of the muscarine in the mushrooms that stimulates the ingester.
Rubbing mushroom juice all over the skinThe method of application that delivers the greatest effects is direct application onto the skin. Again, it is the muscarine that is the active chemical. It excites the muscarinic receptors (acetylcholine receptors) that send electrical signals from the brain to the muscles. The messages become amplified, giving the recipient greatly increased strength and stamina.
Muscarine is soluble in water, and even more so in alcohol. It is likely that the berserkers would have stored the whole mushrooms in a cauldron or vat, perhaps underground, the mushrooms covered in alcohol or perhaps even salt water, to combat putrefaction. When the time came, they would take out a mushroom and rub it over their naked skin, covering every part of themselves. They would then have replaced the mushroom, maybe taking a bite out of it first.
The berserkers were said to bite their shields - illustrated by the berserker chess piece from the Isle of Lewis set. Perhaps this was symbolic of their biting the mushroom shield that covered their bodies. Maybe this is also where the phrase "bite of the cherry" originates. Fruit and nuts were often cited in folk tales in place of the fly agaric, most notably the apple in the garden of Eden, and the hazelnuts of wisdom of Irish lore (Wilson, Peter L. Ploughing the Clouds. San Francisco: City Lights Books, 1999. Print.)