As I've stated in some other posts tonight, it seems we learn something new about Methadone fairly often considering its advanced age in the narcotic community. Dose conversion from any narcotic opioid to Methadone is a difficult exercise in that Methadone appears to have very interesting properties that seem to indicate it has effects on its own potency, and it can effectively be substituted for many other opioids at much lower doses than first thought was the case. All of that is complicated by Methadone's lengthy (and highly variable from person to person) half-life.
First, I'll say that oral/insufflated ingestion of 80-240mg oxycodone/day with at least two "clean" days per week is a relatively small habit. Small on what scale? Small enough to be concerned that you aren't substituting a worse habit in an effort to detox from the original habit.
In any circumstances other than the ones you state here, e.g. impending circumstance in which one needs to perform at one's intellectual and/or physical best, I would seriously recommend that you attempt a cold-turkey kick. The xanax would help you a lot in such an endeavor, along with some OTC meds and supplements. But we won't go down that path since it's not feasible to be in the height of an agonizing narcotic withdrawal if you want to be anything more than useless in any sort of professional or student capacity. However, I will make one final suggestion along these lines -- you might consider simply continuing your Oxy habit as minimally as possible until you get through your exams, and then quit cold-turkey when you have 2 weeks or so to recover fully. If it's possible, I would give it serious consideration.
The next best solution in your situation would be a short term of Suboxone/Subutex (buprenorphine). In case you don't know, buprenorphine is a narcotic similar to both Oxy and Methadone, but with various properties that make tapering and eventual cessation of the medication less agonizing. Unfortunately, you would have to find a Suboxone doctor on a student budget/insurance plan in a limited amount of time, and get started on treatment, all the while (I assume) you would probably like to get some studying in. Understandable. However, if Suboxone is available to you, I recommend it, and like the above plan, I could offer a plan to get you wherever you wanted to be with appropriate supplemental meds/supplements and advice.
However, since you asked, I'll answer: my last choice in this situation would be Methadone due entirely to the potential danger of ending up with a habit worse than the one you have now, with a higher tolerance to go along with it (which naturally means it's more expensive daily). But since it's what you seem to have available, here goes:
Methadone has a very lengthy half-life compared to the opioids you're taking now, or indeed many others. When one is taking Methadone daily, it takes 5 days for the full effects of his daily dose to be felt/appreciated, since some fraction of day 1's dose is still psychoactive inside your brain/body at the end of day 5. What does that mean? Well, it means that unless you plan to do what I'd be worried about above and end up with a bigger habit than the one you started with, you can't expect to feel 100% good on day 1 or day 2, or possibly even day 3, since the methadone is "building up" in your system during that time. What you don't want to do is continue taking oxy for that period, or you will simply lengthen the time it takes to stabilize on a dose, and you want that dose to be as small as possible to reduce the primary danger I've probably overstated by now.
For your habit I'd probably start at 5mg/day and see how you feel on the third day. If you are feeling any physical pain, or intense discomfort with respect to your internal temperature (e.g. cold/hot flashes, goosebumps, all when the external temperature has not changed at all) on the third day, I would increase your dose to 7.5mg. If you're still not 100% by day 5, go ahead and increase to 10mg, but don't go any higher than 10mg/day unless you're actually taking more than 240mg oxy/day.
Xanax won't really be necessary with Methadone -- Methadone is a full agonist opioid, so after your build-up period, and if you're on the proper dose, while you won't be "high" or euphoric, you definitely won't feel even a hint of withdrawal or the negative psychological symptoms that accompany it, e.g. dysphoria, lack of motivation (beyond normal laziness
), etc. If you have trouble sleeping the first night because you haven't built-up the Methadone yet, go ahead and take 0.5-1.5mg Xanax or whatever is a lowish sedative dose for you so you can get a few hours of sleep, but you should be sleeping if not 12 hours of lazy comfortable sleep, then at least 6-8 hours by the second night.
Once your exams are over, I would immediately attempt a taper down as follows: If you're on 10mg/day, taper to 7.5mg/day. If you're on 7.5mg/day, taper to 5mg/day. If you're on 5mg/day, taper to 4mg/day. Hold the tapered dose for a full week (again, it takes a long time for the full effects of the lower dose to be felt, since the day you taper, the previous 5 days' respective fractions of the higher dose will still be psychoactive) and see how you feel. If you're in withdrawal you can't handle, increase 1mg every 2 days til you're comfortable, hold for a week, and start tapering down 1mg/week. If you feel okay, repeat the taper with the same rules -- if you're above 5mg, take off another 2.5mg; if you're at 5mg or less, take off 1mg. Wait a week. Rinse, repeat. This should get you down to the 2-3mg level.
When you get down to 2-3mg, the next taper may have you pretty uncomfortable, because your brain is going to have to start making up the difference in endorphins that it hasn't had to produce since your habit got its legs. In that case, increase 0.5mg and wait to be 100% comfortable again before restarting your taper. The idea is not to increase to the point where you're 100% comfortable immediately -- you want to split the difference; get the brain making those endorphins again, but not be in so much agony you can't sleep, or, worse, you go out and score and have to start over.
If the above DOES happen at the 2-3mg mark, slow down your taper -- 1mg every two weeks instead of every week. Since the plan only allows you to increase half of what you took away for comfort's sake, you have nowhere to go but down, and you'll eventually be clean.
However, the last few mg of any Methadone taper are the absolute worst, and you have to play them by ear and listen to your body. Maybe you want to start tapering 0.5mg every 2 weeks for the last 1mg. Maybe when you get down to a 0mg day you want to start taking 0.5mg every other day for 2 weeks, then maybe every 3 days until you feel like you can stop. You have to play it by ear.
However, I'm banking on the fact that you won't have been on Methadone very long at all when you start your taper, so you will be able to get away with a lot more than, say, me -- I have been taking 110mg Methadone/day for over a year now. I could probably taper relatively quickly (say maybe 5-8mg/week) down to the 30-45mg mark, but after that it would get exponentially harder and I'd have to slow way down; when I got down to the last 15mg, I might easily down to 1mg/month, and the last 10mg might take me a full year. Since your dose will be much lower and your total duration will be much shorter, you shouldn't fall into that necessity, but a word to the wise, you'll be making the transition to that boat if you increase your dose past 10mg/day or if you stay there for more than a week or two at the outside.
Xanax and other benzos are great for the final portion of the taper if you're like me and the WORST part about the taper withdrawal is the insomnia -- just don't trade an opioid habit for a benzo habit; benzo withdrawal can be fatal. I wouldn't use benzos to sleep more than two nights in a row, with optional lower doses to relax if anxiety is an issue; if you're taking some dose of a benzo every day for 3-4 days though you're probably tapering too fast and you should up your methadone a bit in order to lay off the benzos.
If you have intestinal cramps/diarrhea you should take Immodium -- it's a very powerful full agonist opioid on the same level as Fentanyl, but it can't cross the blood-brain barrier, so it won't affect your withdrawal. What it will do, though, is bind to the opioid receptors in your intestines, which gives you a world of help when it comes to the GI withdrawal symptoms.
Warm baths help with pain/anxiety but again if you're suffering either of those to the point of agony you're tapering too fast.
Take a multivitamin daily!!!! Will your withdrawal be much worse or much longer without it? No, but when you're withdrawing, doesn't every bit of comfort and every extra second count? Your brain is going to be manufacturing endorphins in quantities it hasn't needed to since the first dose of your habit. The other physical symptoms are going to be taxing the hell out of your body. You're not going to feel like eating, so at least take a multivitamin to take care of the purely physical metabolic needs that you will have -- this is a very traumatic event for your body and especially your neurochemistry, so give it as much of a break as you can.
I've found that Gabapentin (Neurontin) really helps with certain withdrawal symptoms. It's an unscheduled (non-narcotic, non-addictive) medication sometimes prescribed for nerve pain or seizures. You need a script for it, but if you have a sympathetic doc that knows what you're going through he'll write it no sweat.
Similarly, Clonidine, a blood pressure medication, happens to relieve quite a majority of withdrawal symptoms from anxiety to insomnia to cramps -- you definitely won't feel 100%, but it definitely helps. It's also Rx only, but again it's non-narcotic, uncontrolled, and non-addictive, so any sympathetic doc will write for it no problem.
I seriously doubt you'll need really anything but the Methadone, while you're on the Methadone, and you'll only start needing the other stuff when you get below a certain level of your taper.
If your goal is to get through exams with the methadone and start using again, I'd still recommend tapering with the methadone as low as you can go and waiting at least 36-48 hours between your last dose of methadone and your first dose of recreational opioid -- the methadone you'll be using for exams is going to raise your tolerance, no two ways about that, so you'll want to do everything you can to lower it as much as possible before you get back in the habit, so to speak.
Good luck; all the best.