The Harry Potter star, 57, said his life “changed” 22 years ago, after he admitted he needed help.
Posting a picture on Instagram, he wrote: “My beautiful wife Emma and I in September 1998 looking pretty happy. We weren’t.
“Three weeks later, on October 5th, 22 years ago today, I managed 24 hours without taking any drugs or drinking alcohol.
“I’d tried for decades, but was too smart, too capable and too successful to pull it off.”
The British star, who played Lucius Malfoy in the boy wizard film series, wrote: “Only when I admitted that I needed help did my life change. Thank you to every addict and alcoholic who’s ever lifted me up. I love you all – especially the mad ones. Which is most of you.”
And he added: “There’s a few 24 hours under my belt now, but it’s only ever one day at a time.”