Ryan Leaf Says Public Shaming At '04 Boxing Event Triggered Rx Pill Addiction

Ryan Leaf says his battle with prescription drugs all began back in 2004 ... when he was booed and jeered so loudly at a big boxing event, it sent him into a deep, downward spiral.

The former San Diego Chargers quarterback opened up about his struggles with addiction on Rex Chapman's "Charges with Rex Chapman" podcast this week ... explaining in detail the horrible night that hooked him on Rx pills.

Leaf says he was at a fight in Las Vegas (he claims it was a Mike Tyson tilt, though it seems he has his boxers mixed up because Tyson didn't fight in Vegas in '04) ... when the PA announcer began rattling off the list of celebs in attendance.

"They announced Michael Jordan, Tiger Woods, Charles Barkley, Dr. Dre," said Leaf, who was 27 years old at the time and had just retired from the NFL.

"The audience cheered and clapped. And, then the MC there at the MGM Grand announced my name and like the whole auditorium just booed and hissed."

Leaf continued, "My addict brain heard not only are you a terrible football player but you are an awful human being."

Ryan says that negative reception had a huge impact on him ... and when he was offered the prescription pain killer Vicodin to mix in with his booze later in the evening, he jumped at the opportunity.

And, that's when he says his fight began.

"This would be the first time I abused it and ... I didn't feel that judgment, I didn't feel that fear, I didn't feel that less than," Leaf said. "I didn't feel anything."

"I didn't feel better but it turns out I was just searching for that feeling of not feeling any of the feelings that I had been feeling for so long. I just wanted to numb out. And, that night changed."

Leaf says he went on to fight an addiction with the prescription pills for the next EIGHT years ... as he says he was "chasing that initial high from that night."