Methadone is a very long-acting opiate, useful for chronic pain relief.
It also has less of a ‘high’ than heroin, morphine, oxycodone, and other popular drugs of abuse, in part because of its long-acting nature. The euphoria of methadone is far less than that of the other opiates mentioned above by me. In fact, one hallmark of the methadone addict is that they’re often coherent enough to actually engage in goal-directed activity, like cleaning, shopping, or even working, while on the drug.
As a result, one may get an opiate addict to be a semi-productive citizen while maintained on methadone. I actually know of a surgeon who practices (with the blessing of the Medical Board) while on methadone maintenance (I’d never go to him).
Also, methadone has some utility in discouraging use of those other opiates, if a person is already on high-dose methadone, as the euphoria of heroin, etc. will be severely blunted.
It’s also useful in detoxing someone from opiates slowly, with reduced side-effects.
Even so, methadone is abusable, easy to overdose and die with, and takes a special license to prescribe if you’re using it to treat opiate addiction.
As for controlled heroin doses, well, an active addict tends to escalate the dose to try to recapture the euphoria that he recalls so well from earlier using experiences. That’s how heroin addicts end up on massive doses of the stuff (or dead, or both). And that’s why few heroin addicts stick with scheduled, fixed doses of the stuff.
I’m not a fan of methadone maintenance; it generally requires very high doses to adequately block the effects of heroin or other opiates, and methadone is a bitch and a half to detox off of at those levels, far worse than heroin, frankly. That’s mainly because it takes weeks to months to detox from methadone, rather than the 3 or so days it takes for heroin. And the withdrawal symptoms are pretty much the same.
But methadone is very useful for treating some nasty malignant pain syndromes.