Ex-classmates of Justin Mohn -- the man accused of beheading his dad -- claim he changed for the worse after using LSD ... something Dr. Drew and Dr. Oz view as a very real possibility.

Dr. Drew tells TMZ that if it's true that Mohn was a regular LSD user -- as has been suggested by former associates of his from his youth -- then he's not surprised by the gruesome act that followed. If anything, it only confirms what Drew already knows.

As he explains to us -- LSD and other hallucinogens can be incredibly dangerous over a long period of time ... something he has first-hand experience having worked in facilities that deal with patients going through exactly these sorts of problems.

Dr. Drew says LSD can alter a person's makeup and identity ... including extreme personality changes, mood disturbances, and inability to take care of themselves, among other issues. Drew goes on to explain it can also lead to major psychiatric problems, like personality disorders.

While he doesn't know for certain whether that applies to Mohn -- the crime in and of itself certainly seems to point to that being the case ... and he says it jibes with other instances he's come across in his long career in medicine.

Drew tells us LSD is also responsible for a series of other problems, including chronic psychosis -- which he says Mohn could've been suffering from -- adding at 32, he was at an age where schizophrenia can develop and LSD only exacerbates the condition.

That's why Dr. Drew is adamantly against people using LSD ... he issues a warning that should be followed by any recreational drug users -- namely, stay away!

Drew says for people who get consumed by LSD and end up committing crimes as a result --- as some suspect is the case with Mohn -- the legal system has a way to deal with them.

While Dr. Drew encourages people to give up LSD completely ... Dr. Mehmet Oz, Global Advisor of iHerb, is a little more open to its use, but only if administered and taken properly ... as he, too, warns it can seriously screw up a person if they're just doing it willy-nilly.

As we've reported, Justin was arrested Tuesday night for the murder of his dad, Michael, and for possessing an instrument of crime in the Middletown Township home they shared.

He was arraigned Wednesday, and he's being held without bond. His next hearing is Feb 8.