Heroin was an illegal drug derived from opium which is typically injected intravenously.
Before Day 3
Addicted, Jack prepares to take heroin during Day 3
While undercover with the Salazars, Jack Bauer began using heroin to blend in with the drug cartel. However, he became addicted to it, and continued to use heroin, even after Ramon's arrest.
Day 3
During the events of Day 3, Jack battled his heroin addiction. Also, the Cordilla virus was assumed to look like a crystalline powder, like heroin or cocaine.
After Day 3
Jack's addiction was eventually used as a scapegoat for his termination by Erin Driscoll, who replaced him as Director of Field Operations with Ronnie Lobell.