A month, or so, ago I was a Heavy user of cannabis, tobacco and alcohol, and would have argued vehemently to legalise the cannabis as all I wanted was to be left alone to essentially self destruct. Happily I had a moment of clarity and realised what I was doing to myself and have since stopped all of it. This does give me an insight into just how easy it is to justify its use. Why shouldn't I get high I'm doing nobody harm? This is true especially when compared to alcohol and that's legal. But it does do harm, it's just much more difficult to spot. As Santalum points out motivation is stripped from the user and so has to have a detrimental effect on society in general. Perhaps more harmful is that its effect on the brain can be associated to happiness in the user, as such is so much more addictive IMHO (and much much more difficult to give up) than other drugs including tobacco and I've used lots of drug types.