As other posts correctly identified, the "Chinese medicine" given to Ragnar is betel nut. It is traditionally chewed in East Asian cultures. It does in fact cause reddening of the teeth. However, as it is prepared in the show, it is wrapped in its own leaf. Betel nut, wrapped in betel leaf, is a combination called Paan in some southeast Asian societies. Paan has stimulant as well as psychoactive qualities which explains why Ragnar was hallucinating and acting so bizarrely. Another post identifies the reaction as a "goof," but Paan is in fact psychoactive.
Harald Hårfagre (Harald Fairhair, called 'Finehair' here) is considered the first king of Norway after winning a great naval battle in Hafrsfjord outside of Stavanger, Norway, uniting all the petty kingdoms into one. A great monument is in place there today, called "Swords in Rock" (Sverd i sten). This didn't occur until between 872-900 though, long after the time-line here.