Play video content EXCLUSIVE TMZ.com

Larry King saw some smoke he liked, decided to take a puff and it was a horrible idea ... according to the ex-cable news guru himself.

We got Larry leaving Craig's Wednesday where he was talkin' possible Matt Lauer replacements -- but quickly got sidetracked by a woman with a vape pen. She told LK it was cucumber flavored -- which was more than enough to peak Larry's interest.

Just one puff is all it took to launch him into a coughing fit. Watch what happened when she tried to talk him into puff #2.