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 with Entertainment Weekly. "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 who she is, so it was only a matter of time before she really owned that, and I'm proud of her."

And the Jealous singer admits that while he doesn't keep in contact with Miley, he continues to heap praise on her from afar, adding, "I don't have a relationship with her now really, but that’s not because we have any bad blood - we just sort of run in different circles. It’s a lot like watching your high school crush do well, just on a bigger scale."

Nick and Miley dated on-and-off from 2006 to 2008. Nick also romanced another Disney star, Selena Gomez, while his brother Joe dated Demi, who was on tour in South America with the Jonas Brothers when she suffered a backstage meltdown which led to her rehab stint.