Look at other countries and you can see that decriminalising it has no positive impact on the actual addicts.That's not true though, they're not in prison. Though I've heard prison in Austria isn't as draconian as American prison.
But decriminalization is a half-assed solution. I think it is better than prohibition, because it doesn't result in treating the ill like criminals and doesn't criminalize responsible experimentation, but it only addresses a few of the deficiencies of prohibition.
We need legalization. This will help to combat the illegal distribution chains, which is where most of the real malice is found. It will also give the public a direct means toward helping addicts by regulating the drugs themselves. Regulation can combat overdose by having standard dose sizes so that users know exactly how much they are consuming. It can also guarantee purity — most of the damage to health that a lot of drugs cause, especially meth, are due to toxic impurities.
And instead of shoving addicts into rehab, we should accept that not everyone wants sobriety and you simply can't force it on them because they'll go right back to doing what they want. If you want to address the drug problem you have to accept that. Don't make people do things they don't want, instead, offer them the opportunity to do something they do want. Addicts want to have jobs, or at least some of them do, in order to pay for their drugs. So offer a program to help addicts manage their addiction so that they can be effective employees, and then just give them a job, without the pretense of helping them stop their usage. If you can get addicts back into society, and put them in a healthy environment where they have stability and a purpose, eventually some of them will want to expand on that positive part of their life and might even decide to go to quit completely. Even the ones who don't choose to quit, at least they're being productive, and they're in the care of experts on drug addiction. And the addicts who have no interest in a job will be affected indirectly as well, because addicts associate with other addicts, and if you can get a lot of addicts off the street, it will change the dynamics on the street as well. You can curtail the camaraderie between addicts, who's reciprocity enables them to have a stable source of dope without a stable source of income.
These programs would be expensive, but they can be funded by the money not going into enforcement of ridiculous laws and the taxes from selling drugs.