This comes down to freedom, do you as an adult have the right to put into your body whatever you choose. If you say yes, then drugs should be legal, if you say no then they should be illegal.
As an I adult I feel you should be able to do with your body whatever you choose, now that said if you commit a crime while high you can't use diminished capacity as a defense, after all you took the drug knowing it would do something to you. If you hurt someone, kill someone, rob someone, then you can't say "I was high" or "I'm addicted" as a defense or to lessen the punishment. If you want the right to take whatever drug you choose (alcohol included) you also have to take responsibility.