KJ Apa says he once ate fast food he'd urinated on

KJ Apa says he once added his own secret sauce to some late-night McDonald's ... he urinated on his fast food order ... and still ate it!!!

The "Riverdale" star was talking to Dylan Sprouse and Brendan Columbus on their "Wildmen" podcast about sobriety and addiction ... revealing he will "abuse" anything that makes him feel good ... from substances to food.

KJ launched into a story about a time when he was trying to be sober and simultaneously lose weight for work ... which he says makes him do "crazy s***," like order a mountain of Mickey D's.

He explained he pounded half the order -- 3 Big Macs, McNuggets, fries and a shake -- and instantly regretted it.

KJ says he took the rest outside and peed on it so he wouldn't eat any more ... before purging the half he'd already eaten.

Problem is ... KJ says he was so hungry after purging that even his own piss wasn't enough to stop him from going back outside to polish off the urine-soaked leftovers.

This is a story you just have to hear for yourself ... check out the clip to hear KJ tell it in all its glory.

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