 FACT: SPOILER: The drug used by the twins to poison others is said  to be extracted by a plant called digitalis, supposedly from South  America. Digitalis purpurea, the common foxglove, is in fact  European (particularly common in what is now the UK) and was well  known in the middle ages as a heart stimulant. Being a stimulant,  its effect on someone who ingested it would also not have been  paralysis, as stated in the episode.