I am just careful in this area because previous policies have not been based on scientific evidence, but political agenda (as expanded upon other-where in this thread). So I don't trust my common sense... I accept that different substances have different effects.

Harm is a domain that is very broad and hard to quantify fairly I think. Does Meth cause more harm because more Meth addicts have fallen out of society than say Coke? If so, how can we make sure to differentiate between mere correlations and causal relations?