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I think Methadone is quite an inconsistent drug, especially for the opiate naive. I've had absolutely dreamy fuzzy loved-up days on the stuff with no drowsiness, then a dose double that size has done nothing except made me feel a bit itchy and annoyed. I know that's true of all drugs and opiates to an extent, but Meth is a bit of an odd one.
For that reason I'd err on the side of caution. I think 10mg of meth would be a decent-ish high for someone who doesnt do a lot of opes, wouldn't go higher than that to start.
Last Xmas eve my bro did his first dose of methadone, about 15mg (he does H occasionally) and he spent Xmas morning agitated, annoyed, slurring, dry heaving and wishing he coulda just stayed in bed instead of, y'know, eating lots of roast food and having to talk to relatives we can barely remember the name of. Lol!