EXCLUSIVE

Johnny Lewis was a good guy whose life changed forever earlier this year -- after a very bad experience with drugs ... sources close to Lewis' family tell TMZ.

We're told the "Sons of Anarchy" star was "mentally healthy" --  until an episode early this year, when he dabbled with an undisclosed drug.  Our sources say the incident triggered a "psychotic break" and he never recovered from it.

Family sources tell us ... after the episode Johnny changed dramatically -- he became mentally unstable and began to "self-medicate" to deal with the problem.

The family says doctors told them Johnny's condition was classified as "co-occurring disorders."  Translation -- by abusing drugs to treat the mental condition, Johnny was simultaneously troubled on 2 fronts.

As for the nature of the mental disorder -- family sources say they were never able to get Johnny stable enough to get a clear diagnosis.

As for the landlady's murder -- they do not believe it was triggered by a drug-fueled rage.  We're told doctors had given Johnny medicine to stabilize his condition and when he was taking it he seemed fine, but when he got out of jail 6 days before the murder, they believe he stopped taking his meds and became extremely unstable and violent.