David Sheff says he was addicted to Nic's addiction.

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Looking back, David says he realizes he was addicted to his son's addiction. "When he was on the streets, I was consumed with Nic," he says. "I was obsessed with him to the point that I could barely function."

Over time, David says his obsession put a strain on his relationships. "It was at the expense of my wife and my beloved little children, [but] how could a parent whose son is in mortal peril like this think about anything else?" he says.

David wasn't the only one worried about Nic. Karen, his second wife, their young children, Jasper and Daisy, and Nic's mother, Vicki, also suffered. "Karen was in tears," David says. "For Jasper and Daisy, they were very young, and they revered Nic. He was their big brother, their friend, their playmate. ... They didn't understand what he was doing to himself, and they didn't understand how a family had changed."