The risk of infection is high, and abscesses are guaranteed due to the additives. It will also damage the muscle directly (fibrosis) and this is irreversible. Unlike when methadone liquid is injected into a vein, when it is injected into a muscle it has no where to go. Foreign contaminants introduced into the circulatory system are filtered by the body to a degree, but when introduced into the muscles, they get trapped.
Yes methadone is water soluble, but so is alot of the other stuff in methadone oral solution, so there is no way to separate just the methadone. I used to inject (IV) that red methadone liquid years ago, and it is very caustic. Destroyed a bunch of veins. Putting that into the muscle will do the same, but worse.
Plus even if it was safe it's not really worth it. IV methadone really isn't that much better or stronger than oral methadone, and IM methadone will be even closer to oral in terms of effects. Not worth the dissolving your muscles or risking amputation just for 10% more of an effect
The only time IV methadone seemed really different to me was when I was hospitalized after breaking a bone and had to have surgery. They gave me my entire methadone dose at the time (which was a massive 380mg/day, a dose I was on before breaking that bone) intravenously -- which is something my methadone clinic told them to do because of course my normal 380mg/day orally didn't help my pain really. The thing is, I was used to taking the methadone in divided doses (240mg in the morning and 140mg in the late afternoon). I guess they didn't know that, since they gave me the whole 380mg intravenously. Never been so high on methadone in my life. I almost told them it was too much because I was nodding uncontrollably and it felt like I had to remember to breathe. But being a junky, I didn't tell them I was worried. I figured that if I stopped breathing for long enough my blood oxygen meter would tell them anyhow
I think I only injected methadone when I was on lower doses, like below 150mg. If you feel like you need to inject it then you probably need a higher doses. But if you do inject it, you won't get high anyway, so it's not worth the risk.