The 'Iron Man' star - who was arrested and attended rehab several times between 1996 and 2001 - insists he had always promised himself he would never try class A narcotics and only realised he had a problem when he used drugs to get rid of his "flu"-like symptoms.
He said: "That first time it was opium. The second time it looked like opium. Looked the same, smelled the same, a little dirtier, not quite as pristine a buzz and by the time three weeks later, when I woke up, thought I had the flu and took a hit on it, I looked up and said, 'Great. So now we're junkies. This is f**king great.'
"I was always the guy who was like, 'No heroin. No crack'. But it doesn't matter if you go 10 years without doing it. Because of that 3,551st day, it's your turn."
The 'Sherlock Holmes' actor also admitted he had struggled to eventually let go of his addiction and found himself going back to the habit time and time again.He added to Britain's Esquire magazine: "I'm Retread Fred, serial relapser."