Methadone withdrawal is one of the hardest conditions to treat when it comes to drug detox. The reason for this is because the drug is so sticky in terms of the half life and how long it stays in the system. It is a very powerful medication that leaves the body very slowly, metabolizing about halfway every 36 hours. This makes for a very slow and agonizing withdrawal if you stop taking the medication cold turkey.
The solution to this is obviously to taper yourself down as slowly as possible. If you are taking a large dose every day then you are going to need to come down very slowly in order to avoid being sick. Going to drug rehab will not really help you much in this case as they will not be able to do anything for you other than let time pass so that the medication can slowly leave your body. Therefore the responsibility is all up to the addict to get themselves tapered down to an acceptable amount.
If you do want to go to a drug rehab, the best case scenario is to get yourself tapered down to about 10 milligrams per day or so before you go in so that they can properly detox you once you get there. If you are still taking, say, 100 milligrams per day when you go to rehab, they will not be able to treat your withdrawal at all for the first few days, and by then you will be in residential treatment and attending groups and lectures. This just makes for bad timing because they always put you in detox first at a drug rehab, and then transfer you to the groups and the program once you are detoxed and feeling a bit better. Well with methadone you might not even start to detox until the second or third day, and by then your funding for detox has run out and you will be forced to go to residential and attend groups.
So the bottom line is that if you want to go to drug rehab in order to deal with your methadone addiction, realize that most of the responsibility for detoxing from the drugs lies squarely on your shoulders before you even get to rehab. If you wait and keep taking large amounts of methadone right up until you get to rehab, then they will not really be able to help you with your withdrawal because the drug is too powerful.
Finally, recognize that there can be some potential for danger or even death when going through withdrawal from Methadone at high doses. If you have been taking an extremely large dose for a long period of time, then stopping cold turkey could actually be fatal in some rare cases. So it is nothing to mess around with and the ideal scenario is to get into a medical detox of some sort.