Law enforcement sources tell us a 911 call came in at around 10 PM Tuesday ... the caller reporting a stabbing victim. Paramedics rushed to a San Fernando Valley apartment where they found Ethan Farmer bleeding profusely.

We're told Farmer -- who's also toured with New Kids on the Block -- had eaten either pot brownies or cookies and went off the rails, stabbing himself all over his body.

Turns out, the 42-year-old had some friends over and they were chowing down on edibles when Farmer suddenly turned violent on himself.

He was taken to a hospital where he remains as of Friday night. We don't know his condition.