*Alcohol taken responsibly is not an issue. It isn't illegal, its manufacture is strictly controlled, it isn't made and sold by criminals, and no one is exploited/harmed in its manufacture.
Cannabis cannot be taken responsibility as in most cases you don't know what's in it or how it was produced and distributed.*
Any drug dealt with responsibly is not harmful, even opiates (think of opium based drugs like heroin, morphine etc)
Research by the independent scientific society has produced a harm score in relation to drugs and how harmful they are, this is based on availability, cost harm to user etc, alcohol has a score of 72. Cannabis has a score of 20, meaning that the independent scientific committee recognises alcohol not only as three time more harmful than cannabis but also more harmful than crack cocaine and heroin.
www.thelancet.com/journals/lancet/article/PIIS0140-6736%2810%2961462-6/fulltext
Just because you know where something comes from doesn't mean it isn't potentially damaging - i.e. Magic mushrooms you pick yourself can cause massive harm - just because you can trace them from ground to your mouth doesn't mean that they are good for you.
However on that same point Cannabis now is normally taken in its organic form.
Don't get me wrong I'm against drugs where the affect the user in an adverse way, however I would point out it is rare to see stoners in a mass brawl in town centres at the weekend after kicking out time.