There are some very potent plants/seeds/fungai that can be very deadly, one issue is the dosing. Even within the same species, some specimens will have more of the active compound then others. So if I'm researching something like this, I look up what the active ingredient is that poisons you, see what the lethality of it is, then see how much is in the plant.
Taste will also be something to consider and may cause more nausea. Masking it somehow would probably be a good idea.
Then the side effects. From what I've read, some are definitely not peaceful. Although usually it's abdominal pain, vomiting and diarrhea. Basically whatever your body can do to rid of the poison. Which circles back to my first point of dosing. Whatever the lethal amount is, triple or quadruple it lol.