The results of legalizing drugs have been argued about I guess by millions of people, I say if you like something do it you're only going to live once so if you like let's say cocaine use it, of course some people can't handle it and end up becoming addicts who can't live without it, that's their problem, I was an alcoholic and I quit when my liver started hurting. And if it turns out, like in the case of marihuana that it's also good for you, better yet. And like @onlyvoluntary says, you own your body so you should be the only one responsible for what you give it.
Then another argument is drugs are the reason for many crimes and other unhappy circumstances, which is probably true, but so is alcohol and yet it is legal.
Everyone should be responsible for what he does, for example I have a problem called elemental tremor where my hands and other parts of my body shake, and sometimes I get a sudden spasm, because of this I don't use weapons or drive cars, which I guess is using common sense.