Replacing one addiction with another isn't a cure in patient terms, at all. But having an addict on a legal substance obtainable on prescription means a *lot* of money (hundreds per week per addict) no longer flowing to drug dealers and manufacturers. Furthermore, it's easier to keep track of the amount of [heroin or methadone or whatever] that is being used per day if it's being doled out by prescription and taken under supervision, than it is if you're just taking an addicts' word for it how much they've used. Knowing a true rate of usage is important for treating an addiction. Plus, from your perspective, if the drugs are illegally obtained then you only have the addicts' opinion for it about the amount, strength and "additional content" of what is being used if/when the user becomes extremely ill and needs emergency care.