I started seeing Daniel in June 2005 at a small urgent-care clinic I run for mostly injection-drug users on skid row. An addict, he was a tall, thin man with long, stringy gray hair and an unkempt beard. My notes from our initial conversation described him as "disheveled, dirty and lethargic." On subsequent visits, he often had food or vomit in his beard and would come to the clinic smelling of urine. He seldom had much to say.

Then, in May 2006, I saw a different side of Daniel, whom I will refer to only by his first name to protect his privacy, although he has agreed to let me tell his story. During one of his visits to the clinic, he kept his head down and tears welled up in his powder-blue eyes, but he talked as he never had before. Things weren't good. He'd been jumped the previous week, he told me in his Kentucky drawl. He hated who he'd become, he said — a loner, someone who didn't care about others. "I used to play the guitar professionally," he said regretfully. "I was in a band. I had a life."

Listening to him, I had a realization: He was severely depressed. Until then, I'd assumed that he, like so many others on skid row, was schizophrenic. Schizophrenia is frustratingly difficult to treat, but depression has many effective treatments. My realization about his depression also put his addiction in a different light. He might well be using heroin in an attempt to treat his depression.