i think this has been pretty extensively covered
heres what i wrote before though
is there a history/tendency towards psychosis, schizophrenia, neurosis, obsessive compulsive, anxiety etc in your family?
if so or even if not it could be the thc's nature of directly affecting dopamine balance (exactly where schizophrenia etc shows up) and over prolonged usage and/or genetic predisposition it can bring to the surface or aggravate aforementioned conditions.
i speak from experience and adequate research (though not extensive scientific study)
i quit weed after 7 years of everyday usage for this very reason.
Since then i get more done, enjoy my life, am not paranoid, am married and happy with a career in the music business.
Knock it on the head mate is my suggestion, and read up on dopamine and its importance in connection with well-being.
Would it be so bad if you left the stoners on the sofas and got on with your life?
"it's only weed" thinking can often be used to belittle - but for me it is the one substance that sends me round the twist - slowly surely and completely.
Seriously give me any class a over that dreaded weed - (though i can, now, enjoy a mild high coupled with alcohol or benzos - takes the anxious edge off)
take it or leave it, but peace wished upon you nonetheless.
DJ