Nick Jonas has confessed he was "genuinely concerned" close friend Demi Lovato's addiction to drugs would end her life.

The 23-year-old pop stars have known each other since their early teens, and Nick watched the Heart Attack singer struggle with substance abuse, self-harming, and eating disorder problems, which prompted her to seek rehab help in 2010, at the age of 18.

She recently celebrated her four-year sober anniversary, and while Nick is extremely proud of his pal now, he admits he used to fear receiving news about Demi's demise.

"It's been years of working on herself and getting better," Nick explains in a new interview. "She's amazing - and a better singer, too, because she's healthy. But I was genuinely concerned that I might lose my friend."

Nick, who is heading out on a joint North American tour with Demi this summer (16), also gushes about his ex-girlfriend Miley Cyrus, who he dated years ago when both were breakout stars on the Disney Channel.

"I love (her song) Wrecking Ball so much," Nick says. "I still think it’s one of the best pop songs of the last 10 years. And you know, she's always been so outspoken and authentic to whom she is, so it was only a matter of time before she really owned that, and I'm proud of her."