The toxin it produces, ricin, is a protein found in the castor bean, but the poison itself is made from the mash that's left over after grinding the beans into oil.
Mrs. Driffield looked as though she had taken a dose of castor oil and had just been trying to get the taste of it out of her mouth by sucking a lemon.
The extremely powerful toxin Ricin, produced in the seeds of the castor oil plant, can kill you if you inhale even one milligram, which has made it popular for assassination attempts and biological warfare around the world.
Then Votini fell to twirling his very handsome white castor hat on the tip of his forefinger; but the boy—and it seemed as though he did it on purpose—did not deign even a glance at the hat.
Taken orally its LD50 is about 20 milligrams per kilogram of bodyweight, about 8 castor beans worth of the poison, but if it's injected, like the way Markov got it, it's just 1 tiny microgram per kilogram.