Lamar Odom -- I Was DRUNK, NOT High On Drugs

Lamar Odom admits he fell off the wagon last week ... but he insists he was s**faced drunk, and not cracked out.

Lamar told friends he's seen the video (above) we posted on TMZ, showing him slurring his words in an L.A. bathroom as he rapped with his fellow shirtless friend Jamie Sangouthai.

Lamar says it was a one time deal and NO drugs were involved -- he was specific about 2 drugs he's had problems with -- crack and coke.