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. Ricin causes severe diarrhea and victims can die of shock. Death typically occurs within 3–5 days of the initial exposure.
The first time Walt created ricin it was intended to kill Tuco Salamanca. Walt carried a packet of meth mixed with the poison hoping that Tuco would snort it. The ricin made the meth stink like head cheese, which raised Tuco's suspicion about its quality. Jesse said it was a new recipe with his old secret ingredient: chili powder. Unfortunately they learned that Tuco hated chili powder and tossed the packet aside.
Next they tried to get him to eat it. Walt sprinkled the packet into Tuco's food, however Hector Salamanca saw what he did and rang his bell to prevent Tuco from eating. Misinterpreting Hector's warning, Tuco simply traded plates with him. Hector knocked the plate onto the floor instead of eating the poisoned food, thus dispensing with the ricin.
The second time Walt created ricin it was intended to kill Gus Fring. He created a small vial of it in Gus's own superlab, supposedly out of sight, and secretly passed it to Jesse, who concealed it in one of his cigarettes. At the time, however, Jesse was distancing himself from Walt and growing closer to Gus, and thus he never administered it even when he had the opportunity.
Later, Andrea's son Brock fell severely ill from some unknown cause and Jesse found that his ricin-vial cigarette was missing. He told Andrea to warn Brock's doctors about the possible poisoning. Because only he and Walt knew about the ricin, Jesse suspected that Walt poisoned Brock; however, Walt convinces him that it was Gus who poisoned Brock. ("End Times") Although initially in danger of dying, doctors say that he would survive, and tests showed that it was not ricin; instead, he ingested the berries of a local poisonous plant called the "Lily of the Valley". At the very end of the episode, the camera zooms in on a Lily of the Valley in Walt's back yard, beside his pool. ("Face Off")
Saul returned the ricin cigarette to Walt, stating his wonder that Huell didn't break it when he lifted it from Jesse. ("Live Free or Die") Walt hid the ricin capsule behind an electrical outlet in his bedroom and made a false capsule with salt. He hid the fake capsule in a cigarette and planted it in Jesse's Roomba to put Jesse's mind at ease. ("Madrigal") Walt later took the ricin to a meeting with Lydia with intentions to kill her, though didn't because she proved to be useful, by offering expansion of Walt's empire into the Czech Republic. ("Gliding Over All")
- Ricin on Wikipedia