In the 4th season of Vikings there is a new character named Yidu that is presented as a Chinese slave. In the 4th episode of Season 4, for the first time she gives Ragnar a plant that she says is a medicine, but that obviously would be considered a drug in modern terms, because Ragnar gets highly addicted to it.
First I thought it was opium. It would make sense, because opium is mainly produced in Asia (the girl is Chinese), and by that time, 9th century, opium was already well known in Asia as a type of medical plant. Furthermore, Ragnar represents some of the typical symptoms of opium like sweating and the euphoric state.
But, this post here suggests that most likely the plant is Betel Nuts or a Paan blend and it lists a number of arguments why it can be so.
Is there anything definitive that answers this question?