was a mile high during his concert in Colorado Thursday night ... so wasted the cops had to step in.
The concert started out well ... the opening acts were great. But things went South when Ralphie tried to take the stage ... he was so slow it took forever.
When he got the mic things got worse. Audience members tell us he couldn't string a sentence together, couldn't finish jokes, couldn't remember what he had just said.
Audience members started yelling at him to start telling jokes, then demanded refunds. Some die-hard supporters yelled back at the hecklers and it went all Cosby
Ralphie finished the show, but by then cops had come to maintain order.
Ralphie's camp tells TMZ ... "He was like a kid in a candy store, being in Colorado for the first time since pot became legal." They add, "We believe in conspiracy theories, we think Cosby snuck in and drugged him. We're pressing charges because we're within the statute of limitations."
We're told what really did Ralphie in ... edibles.