A suicide capsule was a pill that could be swallowed in order to cause self-imposed death, or suicide. It was issued to CTU agents so that they could take them when captured, in order to prevent intelligence information from falling into terrorist hands.
Suicide capsules were also sometimes used by terrorists once captured by counter terrorist units so that they would not give up information relevant to their plot. They were usually only used by terrorists extremely devoted to their cause.
Second Wave terrorist Mohsen used a suicide capsule hidden in his teeth to avoid revealing the location of Syed Ali. Jack Bauer later tracked down Ali, who also tried to this technique, but Jack forced the capsule out of Ali's mouth so he could interrogate him to find the location of the nuclear weapon he was planning to detonate.
At the Chandler Plaza Hotel, Agent Michelle Dessler witnessed the painful death caused by the Cordilla virus. Between the hours of 5:00am and 6:00am, she asked Tony Almeida to send over some suicide capsules in order to give the infected people the option of dying peacefully. Although Tony was not authorized to do this, he did so with the blessing of Ryan Chappelle.
Between the hours of 6:00am and 7:00am, a case of suicide capsules arrived at the hotel. Michelle told the infected hotel guests that she could offer them a less painful death. She said that the pills would make it feel like they were going to sleep.
After a hint in David Palmer's memoirs led to a man at Ontario Airport named Chevensky, Jack Bauer went to the airport to try to find him. Jack found Chevensky and confronted him. But as he was distracted by an explosion outside, Chevensky managed to use a suicide capsule to prevent Jack from interrogating him.
After he was arrested at the White House for treason, Jonas Hodges was visited by Cara Bowden who was impersonating his lawyer. Bowden, who was sent there by Hodges co-conspirators, gave him a suicide capsule for him to kill himself assuring him they would protect his family. While he was being transferred to the FBI for further questioning, Hodges took the capsule and started suffering a cardiac arrest. However, Sergeant Cadden reacted quickly and he survived, though Hodges was killed hours later by an assassin.