(CNN) Michael Phelps, the most decorated athlete in Olympic history, has a checkered and well-publicized history with alcohol and drug use.
But is the 18-time Olympic gold medalist an alcoholic?
"I don't know" was his candid response when the question was put to him. "I honestly don't know."
"I know I probably have moments where I have gone off the deep end where I shouldn't."
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USA Swimming suspended Phelps for six months in 2014 for his second DUI arrest -- the first coming 10 years earlier -- and he was left out of the U.S. team for the 2015 World Championships.
Shortly afterward, Phelps attended a six-week, in-patient program to "provide the help I need to better understand myself."
Read: Michael Phelps suspended by USA Swimming for six months
"I checked myself in because I think I was at a point in my life where something needed to change," Phelps told NBC.
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In 2009, a year after his historic eight-gold medal haul at the Beijing Olympics, a photo emerged of Phelps smoking marijuana, an incident he said made him realize who his real friends were.
The 30-year-old is training for what will be his fifth Olympic Games -- Rio 2016.
"I want to retire how I want to retire -- and I have a great opportunity to do that," he said.