Well, 3ml of most commercial methadone AFAIK would be 15-30mg(it’ seems to be 5-10mg per ml mostly)
So don’t shoot 120mg, that is insane-Onky becuase opioid calculators suck at methadone conversion, will tell you that CHRONIC methadone is usually about as potent as IV morphine, however cross tolerance comes into play
Pretend it is twice as strong as methadone, make sure you have sitter if inexperienced, and, an antihistamine would be prudent with morphine (Cetirizine is non drowsy and seems fairly benign, if unfamiliar with H1 antagonist medications) - I am going to probably close this, PM me if you have a specific question, or questions, otherwise, vague dosing advice isn’t exactly what we ideally do, please be safe and cautious and feel welcome to ask questions