I don't think someone coming off 90mg methadone cold turkey necessarily has the best advice to offer someone hoping to successfully get off it. I don't mean to belittle w0w's insight into the process, but if you can at all avoid coming off it at a high dose cold turkey you are infinitely better off tapering off your dose at a more reasonable rate. That is how most people are successful getting off it, with higher rates of long term sobriety than those who jump off a high dose.
What does are you currently at?
If you're above 40mg decrease your dose by 5mg a week until you hit 40mg or start feeling minor withdrawals from tapering, then decrease the taper to 2mg a week. When you get down to 30mg or 20mg, whichever dose you feel it would be better to further slow the taper down, decrease your taper to reducing your dose by only 1mg ever week. When you get to 20mg, hang out there for a couple weeks. When you get down to 10mg, hang out there for a couple weeks to further stabilize. Then continue going down 1mg a week until you are around 5mg.
When you get down to 20mg, start working with a doctor outside the methadone clinic to get comfort meds organized for when you come off the methadone. Ideally you would have, at the very least, gabapentin, clonidine and a long acting benzodiazepine like diazepam or clonazepam (those are recognized as the traditional gold standard for methadone detoxes, and there is no doctor worth his salt would wouldn't be happy prescribing you them for when you jump off).
If you could also get a low dose of buprenorphine to take for a week or two after jumping off methadone that would be ideal, but isn't absolutely necessary (it is the new gold standard for methadone detoxes, but most doctors are still too unfamiliar with the medication to understand its significance as part of the detox protocol).
