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) .