It is not a given that anything that is dangerous to an individual should by definition be illegal.

That marijuana may, as you say, not be safe does not mean it has to be outlawed. A lot of things in life aren't "safe." That's part of being free.

ER is right in that we should have learned something from Prohibition, which also generated a huge criminal class.

Marijuana should, in a legal framework, be treated no differently than alcohol (DUI, age restrictions, etc.) There is no rational reason to declare that one should be illegal and not the other.

One history of how the Evil Weed was outlawed (I haven't checked any of this, so take it for what it's worth).

The federal approach is important. It was considered at the time that the federal government did not have the constitutional power to outlaw alcohol or drugs. It is because of this that alcohol prohibition required a constitutional amendment.

At that time in our country's history, the judiciary regularly placed the tenth amendment in the path of congressional regulation of "local" affairs, and direct regulation of medical practice was considered beyond congressional power under the commerce clause (since then, both provisions have been weakened so far as to have almost no meaning).