My ideal system would be one where you can use whatever the hell drug you want in certain monitored and sponsored facilities. You would be able to purchase whatever drugs you wanted for immediate consumption only while you were there (with a lenient certification background check for harder and more addictive drugs, so that you weren't blowing all your money on heroin when your kids were starving, as well as potential age limits for certain drugs just like we do for alcohol as well). You wouldn't be allowed to leave until you passed a test showing that you had regained full control of your mental facilities. You would have your own room you could hangout in by yourself, or if other people agreed to a waiver (prior to them actually using any drugs) they could join you in your room. You could bring whatever the heck you wanted in with you, games, phones, food, whatever, unless they significantly increased your chance of causing harm to to others (power tools or weapons, for example). Quitting and withdrawal treatment programs would be offered to anyone free of charge, paid for by a percentage of the cost of drugs people bought.
Said system would mean that:
1) You could easily obtain whatever your drug of choice was with only a short trip, eliminating the power of drug cartels.
2) Overdoses and bad interactions would be virtually eliminated, since the amount of any given drugs a person could use at once would be monitored.
3) Drug quality could be monitored to ensure all drugs were as safe as we could make them.
4) Should you decide to quit free treatment and help programs are offered.
5) Possible drug certification checks would ensure that you aren't feeding your habit at the expense of those who depend on you.
6) There wouldn't be anybody killing or harming others who didn't agree to the risks because of any drug effects, through any method.
The only con I'm seeing here is that you have to travel a short distance to use your drugs, but with the popularity of some drugs I could very easily see a Facility becoming available in even most smaller towns, meaning it wouldn't be any more of a hassle than doing something like going to the grocery store. (Though some drugs might only be available at larger Facilities). The Facilities would all be self-supported by the prices of the drugs they sell. Simply making things not illegal should cause a fairly significant price drop in most cases, so a fairly high government cut could be placed and still have a much cheaper and more obtainable price for most drugs, so money isn't going to be a problem there. I'd be wiling to bet that they would even make money for the government, similar to the post office.
It'd end up being just like any other form of entertainment, you'd hit up the Facility on a saturday afternoon with some friends, just like you do every week. Each of you already has a waiver signed saying you are fine being in the same room and you wave certain rights while you are present so it's just a simple matter of turning in your keys and pocket knives, subjecting to a quick scanner like we have in the airport to make sure you aren't smuggling anything in, and then you go to your nicely furnished room. You each log in and hit up the menu on what particular drugs you are certified for, entering in how much of each you want (which would be limited by what you chose and your body size/weight to prevent overdoses and bad interactions) and then pay right there. A little while later someone delivers the drugs, stays long enough to make sure that you aren't swapping them around between you, and then leaves. If one of your friends hasn't signed the "I'm okay with death due to violence here" waiver for your room you might have a camera to ensure one of you didn't try to kill someone by accident, but if they all have then you aren't even watched with that. After a few hours of relaxing you and your friends go to leave, take the basic "I am in full control" test which involves only some simple math and word problems, a quick reflexes test, and maybe a few other small, easy to do tests; as well as submit to another quick search to make sure you aren't smuggling drugs out of the Facility. After 5 minutes or so all of you have passed, so you each pick up your keys and pocket knives from the front desk and leave.
Should anyone decide to quit a drug they simply fill out a form (while not drugged) stating that they intend to quit X drug by Date (which would have a minimum if you would suffer from extreme physical withdrawal effects). At that point it would set a cap on how much of X you could purchase over time, which would slowly reduce at a rate that prevented any serious symptoms until you were completely free come X date. You would also be able to sign forms stating that you wanted to be permanently prevented from relapsing, in which case you wouldn't be allowed X at any time in the future either (or you could have lesser forms that simply gave you a warning, etc.). That should result in a treatment program that is basically guaranteed to work if you let it, because it's a hard cap that limits you, comes gradually enough that you don't suffer any serious symptoms, and can prevent relapse, all for free.
And sure, something like certification for individual drugs might seem like it would create some small demand for illegal variants, but with similar penalties on anyone non-regulated who is caught producing or selling drugs, and heavy penalties for people caught using drugs outside of the Facility I'm guessing the cost of illegal variants probably wouldn't drop too much. Why would you bother to pay more for illegal versions when with just a little bit of that money you can clean up your act enough to get certified, then get it both legal and cheap? You could even have conditional certifications where a case worker comes around once every couple of weeks to check if you are meeting your requirements and if so you are still certified, but if you aren't then you are cut off (unless doing so would cause you severe symptoms, in which case you would be quickly weaned off). Maybe even incorporate something like the 3 strikes system, so that if you fail a conditional agreement 3 times than you are no longer allowed to be certified for that particular drug.
For production, it would be carried out by government contracts that would have a fixed price based on the current cost to manufacture a drug (as investigated at the start of each year) + government cut + some profit for the companies that pick up the contract and would translate directly to the cost to any consumers. These could serve as a guaranteed, if slightly low, form of stable income for companies so should almost certainly be picked up by somebody. Contracted companies would then have to submit to both an initial series of inspections, and then random and regular inspections afterwords to ensure that they weren't selling or smuggling their particular drug anywhere but to the government.
The end result should be that people who want to use drugs are free to, in a safer and cheaper way than they are now, and the only rights that people would have to give up are that of using drugs irresponsibly (which we seem to have agreed is something you shouldn't be doing anyways) and a small bit of comfort/hassle in that you are forced to drug in the comfortable rooms at the Facility rather than those of your own home. The drug users get a wider and cheaper variety of drugs, the non-drug users don't have to worry about the drug users hurting anyone but themselves, the people who want to go to rehab have free access to better ones, the government gets a second self-supporting department that is actually operating in the black, the companies get reliable income from government-sponsored contracts, and science gets access to a huge bundle of data on the physical effects of different drugs and how to make them safer or develop new ones. Everybody wins! (Except for the drug cartels, who lose as badly as we agree they should).
And yeah, done with the whole "harm" discussion, we've pretty much exhausted that.