Be careful, as it is a very strong potentiator of opiates- even just a threshold dose (for me, 10mg plugged is about threshold) will cause a noticible increase in the effects of the opiates.
Also- most Methadone is racemic (I believe Levo-methadone is only given in Germany, I haave no idea- 99% of methadone 'on the market' is racemic thuss the following applies), thus is itself an NMDAr antagonist, albeit a much weaker and slightly different type. However, NMDAr Anatagonists are known to often synergise in strange and unexpected ways (for instance tramadol, which is also a NMDA antagonist, + ketamine does weird things to you that morphine + ketamine just doesn't do).
So yeah, you should play it safe with this one and methadone. Everyone who's been at this shit long enough knows someone who has died from methadone- generally because they combined it with something else and then made stupid decisions that resulted in them never waking up. I'm only 24 and I've had 4 friends die from methadone, 3 who had been on methadone 10+ years. One involved ketamine, but then again he took a fuckload of benzos...I don't know what happened in his room that night, but I knew him and I'd say he took his shot his done (he had takehomes, so took his dose in two lots, IV) took some ketamine and blobbed out for a few hours taking a few benzos here (but it all added up...) and there while watching TV before nodding out and at some point he stopped breathing.
Don't be another methadone overdose. Even if you get to the hospital methadone overdoses suck ass because it kicks in so slowly that they have to give you multiple blasts of naloxone. If you're opiate dependent naloxone is the single worst fucking thing that you can be given. Yes, it saves your life, but you are thrown into instant acute withdrawal- yaaay! Seriously, be careful. I'm not trying to be preachy or anything and I do dumb shit like this almost every day, but then again I've nearly killed myself multiple times, I've passed out in my shower, in my bath, woken up on countless floors, I've overdosed twice (incidently, both times on morphine, benzos and ketamine)- once while an addict, once while not. Naloxone, your saviour, is a bitchqueen from hell. And if you overdose it'll fuck up your MMT. Might even get kicked off or forced on to some brutal 2 week taper or some shit. Fuck that.