Ricin is a poison that Walter White prepares twice. Even though it is not the fastest-acting method of killing, Walt prefers it because it leaves behind little or no trace of its use on a victim.
Jesse Pinkman also requested ricin from Walt to kill the rival dealers who made Tomás Cantillo murder Christian "Combo" Ortega, however at that time Walt refused to get involved in that dispute ("Half Measures").
Ricin is a naturally occurring protein derived from the beans of the castor oil plant, Ricinus communis. Ricin is highly poisonous if inhaled or injected, but less so when ingested.
When inhaled or injected, just a few grains of it can kill an adult, due to its high toxicity: refining the pulp of as few as eight beans can create a dangerous amount of ricin. The LD50 (median lethal dose) is roughly 22 micrograms per kilograms in humans. It was famously used to assassinate Bulgarian defector Georgi Markov in 1978 - the KGB injected him with a modified umbrella with a tiny pellet dosed with ricin.
The poison acts by inhibiting protein synthesis in ribosomes, causing widespread cell death in numerous organic systems. It is resistant, but not impervious, to digestion by peptidases.
Ingesting ricin is much less lethal, requiring on the order of 30-40 mg per kilogram. 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 Pinkman 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 "Tio" 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 Gustavo 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 Cantillo's son Brock Cantillo 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 Goodman returned the ricin cigarette to Walt, stating his wonder that Huell Babineaux 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 Rodarte-Quayle 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")
While Jesse was waiting to start a new life in Alaska he put the pieces together that Walt had Saul have Huell lift the ricin cigarette. Jesse confronted Saul, took his gun and Saul confessed. Jesse then stole Saul's car to go after Walt. ("Confessions")
Months later, Walt heads back to his now-abandoned house and retreives the ricin from behind the electrical outlet. ("Blood Money") Later that day, Walt waits at a coffee shop for Lydia and Todd to meet. Walt joins them and asks them for $1 million in exchange for a new technique he knows how to make Blue Sky without methylamine. Lydia refuses and Walt leaves. Lydia stirs a packet of her favorite sugar substitute—stevia—that she found on her table's sweetener holder, into her chamomile tea. That evening, Lydia calls the recently deceased Todd's cell phone and Walt answers. He asks her if she is feeling under the weather and tells her that he poisoned her with ricin, then tells her "goodbye." ("Felina")
- Despite its predominance in the series since Season 2, it wasn't used on a character until the series finale.
- Ricin on Wikipedia