Ricin is a poison. It is a highly toxic, naturally occurring protein. It is produced in the seeds of the castor oil plant Ricinus communis.

A dose the size of a few grains of table salt can kill an adult human.^[1] The median lethal dose (LD₅₀) of ricin is around 22 micrograms per kilogram (1.78 mg for an average adult, around ​¹⁄₂₂₈ of a standard aspirin tablet/0.4 g gross) in humans if injected or inhaled.^[2] Oral exposure to ricin is far less toxic and a lethal dose can be up to 30–40 milligrams per kilogram.

Ricin prevents cells from assembling various amino acids into proteins, and death occurs after a few hours up to a day. Ricin has been used as a terrorist weapon, including the assassination of Georgi Markov in 1978, supposedly by the KGB.

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