I actually researched it and Hydrangea isn't actually that poisonous, the scene in which they show his gramps eating it actually explains a few things if you read this :
Vomiting, depression, diarrhea. Cyanide intoxication is rare – usually produces more of a gastrointestinal disturbance.
^symptoms.
Old geezer probably eats the leaves to clean his ass lol. The process of making that mixture he made was probably needing the mild cyanide amount it has, as any other toxic plant would probably have too much or too little amount, keep in mind this is an ancient document so they probably didn't have a way to extract that from plants like modern medical science can. It can also explain why some of the old toxins and poisons that are renowned as being extremely dangerous can't be compared to modern ones, the ancient ones were far more dangerous because they used different ingredients than what is being used now. Probably assassins and ninjas were this effective due to unique ingredient doses. Modern poisons are less subtle and usually have pure substance in it, while ancient ones had some ingredients that could effectively enhance the effect quite considerably.
I think Rea wasn't as affected from the preview I seen but i dunno, i prefer not to see previews so I just forgot about it. I have a feeling there will be some flashbacks now and we will see if Rea was actually affected in episode 4.
I dunno, at any rate I'll be hunting OP/ED songs when i'm home.
