You can't compare it to alcohol. Cannabis is medicinal with proven medicinal benefits, and alcohol is not. Not to mention alcohol kills people and breaks up families when abused and not taken in moderation. The second someone compares cannabis to alcohol, I stop reading because everything said afterwards isn't credible.
In the interest of fairness, on a recreational use basis, I will gladly compare it (favorably, risk/health wise) to alcohol.
Substance for substance on a chemical level, absolutely no comparison indeed.