Nic Sheff in San Francisco's Haight Ashbury district

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David tried many times to connect with his teenage son, but after years of abusing drugs, Nic ran away from home.

For the next five years, David says his son was on a suicide mission fueled by crystal meth. While living on the street, Nic says he dealt drugs, worked in the sex industry and stole money from his family to support his habit. At one point, Nic says he was so desperate, he came home to search for cash. He found a few dollars in his little brother's piggy bank and stole every penny. "Doing crystal made me into like an animal," he says. "I would have practically done anything to anybody in order to keep getting it."

Nic's addiction ravaged his family and his body. "I was really skinny," he says. "Crystal meth does this thing where you get kind of like scales on your skin."

Even after an overdose sent Nic to the emergency room, he refused to call his father. "I was afraid to call my dad. I was ashamed too," he says. "There was just this idea that I was just going to shoot drugs until I killed myself."

David says he used to scour the streets and search alleyways looking for his son. One day, David convinced Nic to meet him at a cafe to talk. When he didn't show up, David says he found him on the street, strung out on meth and looking very frail. "My heart just sunk," David says. "There was my son, my beautiful boy, and he looked like he was walking dead."