Ricin, from the castor oil plant Ricinus communis, is a highly toxic, naturally occurring protein. A dose as small as a few grains of salt can kill an adult human. The LD50 of ricin is around 22 micrograms per kilogram (1.76_mg for an average adult, around _228 of a standard aspirin tablet/0.4_g gross) in humans if exposure is from injection or inhalation. Oral exposure to ricin is far less toxic and a lethal dose can be up to 20_30_milligrams per kilogram. Ricin is poisonous if inhaled, injected, or ingested, acting as a toxin by the inhibition of protein synthesis. It is resistant, but not impervious, to digestion by peptidases. By ingestion, the pathology of ricin is largely restricted to the gastrointestinal tract where it may cause mucosal injuries; with appropriate treatment, most patients will make a full recovery. Because the symptoms are caused by failure to make protein, they emerge only after a variable delay from a few hours to a full day after exposure.
Contributions by Castoroil101, Dachshund, and Rwendland.