http://breakingbad.wikia.com/wiki/RicinThe 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.
For Gus
Season 4
Ricin hidden in the cigarette that's turned around
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")
Season 5
Saul Goodman 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 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")
In 2010, Walt heads back to his house, now abandoned, and retreives the ricin from behind the electrical outlet. ("Blood Money") 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")
