Knowing how much we love berserkers, here's another berserker theory (https://www.academia.edu/40070517/Sagas_of_the_Solanaceae_Speculative_ethnobotanical_perspectives_on_the_Norse_berserkers), this time suggesting henbane as the magic ingredient. Being by an ethnobotanist rather than an archaeologist or historian gives a different perspective. The weakness, as ever with drug explanations, is there is no evidence of them taking anything.