(CNN) Mark Restivo says he brought in nearly $5,000 a month selling prescription drugs.

"I had to feed my habit as best I could. I was constantly buying and selling and using pills," the former New York police officer said.

"I went from a police officer to a drug dealer within a matter of months," Restivo, 39, explained. "I abused Adderall, cocaine, oxycodone, methadone and suboxone."

Dr. Sanjay Gupta introduces you to people who have overcome medical challenges and, by following their passions, have made bold changes in their lives.

Restivo battled alcoholism and drug addiction long before becoming an officer, he says. He was just 14 when he had his first shot of vodka.

"Looking back, I was an alcoholic. I didn't drink like everybody else did, because for some people, they were able to have a few drinks on the weekend and stop. I just continued to go in, and I never really stopped."

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