The rock icon is candid in the magazine's cover story, which hits newsstands today, adding, "but there was a point where I was the worst drug addict."
"I was a girl, I was fragile, and I was doing a lot of coke," she said.
"I was very worried about her,” Petty reveals. “To the point that if the phone did ring and they said, 'Stevie died,' I wouldn't have been surprised.”
Having kicked the hard stuff, Nicks admits she still smokes a bit, especially with her still active in the music industry today.
"When I'm writing, I will allow myself to smoke a little bit of pot," she added. "It's my one little thing that I can do. I use it as a tool, and I'm very careful, you know? And I get results. However, if I thought it was going to lead me back to something worse, I'd stop."