Heroin was an illegal recreational drug derived from opium, which was typically injected intravenously. It had powerful effects and often led to addiction.
While preparing to go undercover with Ramon Salazar, Jack Bauer started taking heroin in order to convincingly pass himself off as a drug user. He kept using the drug during the operation, and after he arrested Ramon he was still addicted. On Day 3, Chloe O'Brian found evidence of his drug use in his office and informed Ryan Chappelle, who brought in Rae Plachecki to start an investigation. Although he was not removed as a result of the investigation, when Erin Driscoll became CTU Director she fired Jack for his past drug use. ("Day 3: 1:00pm-2:00pm", "5:00pm-6:00pm", "2:00am-3:00am", "Season 4 Prequel")
After the mission to take out Ibrahim Bin-Khalid, Ben Grimes became addicted to heroin. The police in Washington, D.C. busted a heroin deal the week before Day 10,and confiscated a large amount of money, which Eric Carter took from the evidence lockup. ("Day 10: 1:00pm-2:00pm")