Heroin is a highly addictive alkaloid-based narcotic. Its manufacture and use is illegal in most countries, including Australia, the UK, and the United States. It is made from morphine.
Charlie and Liam
Charlie became addicted to heroin after his brother Liam introduced him to it. Liam later kicked the habit after almost losing his family, but Charlie continued to use even after the Crash. Locke convinced him to fight his addiction, first giving Charlie three chances to ask him for his stash before getting it. Charlie asked three times, but on retrieving his stash, he didn't use any of it. Instead, he threw it into the fire. ("The Moth")
Eko, some years earlier in Nigeria, killed two Moroccan drug dealers while trying to smuggle large amounts of heroin out of the country. He arranged to smuggle the drugs out in Virgin Mary statues aboard a plane. The plane took off without him and crash-landed on the Island.("The 23rd Psalm") ("Deus Ex Machina") ("Exodus, Part 2")
When Eko came to the Island and heard Charlie carried one of the statues, he broke one, revealing its contents to Claire. He gave Charlie another from the plane to replace it. Locke believed Charlie had relapsed, and Charlie's visions and odd behavior supported the belief. He confiscated the heroin and kept them in the Swan armory. Charlie helped Sawyer clear the armory, but refused the heroin. Sawyer later gave some to Jack so he could ease dying Libby's pain. ("The 23rd Psalm") ("Fire + Water") ("?")
Vincent discovered the remaining statues and dropped one at Charlie's feet. Charlie threw them into the sea, evidently conquering his addiction, to Locke's approval. He returned to heroin after death though -- police escorted him off his flight after Jack removed a bag from his throat. Swallowing the bag gave him a brief flash of his life. ("Three Minutes") ("LA X, Part 1") ("Happily Ever After")
- Brown sugar stood in for heroin in scenes