alt Hacker NewsThe correlation used to cite heavy (daily repeated administration across the day; what the kids call "the chronic.") But now its just any degree of use.
I should add that I happen to know of more than one heavy user who subsequently progressed to Schizophrenia or bipolar disorders so I don't personally doubt the cause and effect.
But this blanket correlation seems to me to be overbroad.
Marijuana certainly doesn't cause bipolar disorder, but it definitely can affect or potentially induce a manic state. Of course bipolar people are already known to self medicate heavily and use a lot more drugs in general, and drugs are not the only way for them to self induce mania.