Prove it's bad? OK then....
1 - As yet unproven, but seriously strong possibility it can bring on mental illness. Not in everyone of course - just like many people could take ecxtacy and have no effects, but another could take one and die. Want to take that risk - your choice.
2 - Proven risk of mental incapacity through heavy use. Then again, some people haven't many brain cells to begin with.
3 - Proven lung damage, massively increased compared to tobacco, I better elaborate on this.
It's due to two factors - firstly is the fact marijuana burns hotter than tobacco - doesn't take a genius to work out how that can cause more damage.
Secondly, it's the way it's smoked. Marijuana is inhaled deeply and held in for much much longer than tobacco. Tests have shown moderate weed smokers in their 20's have the same lungs you would expect in a 60 year old heavy tobacco smoker.
Harmless? Believe what you want.
As for if it does any harm, well I'd have the law changed (At the moment police in the UK cannot do roadside drug tests - but this is being worked on as we speak) so anyone caught high driving has the book thrown at the bastard
Save it for in your own houses chilling to music or suitable movies guys.
Me, I don't like it, it makes me feel out of control, it makes me feel sick - but partially paralysed so I can't do anything about it.
As for the tired old "Well tobacco is legal" one which people often drag out - well if smoking was invented today, it would be banned, no-one is in any doubt abouut that. It's firmly entrenched however, and the fact governments won't ban it, is because it is so part of society, and the cruelty of banning this habit with millions who enjoy it and are actually hooked on it would be unbelievable. Tobacco smoking is different.