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is a opioid
drug, 3 grams is not a low dose, and you are taking it since last 7 days, in usual conditions Urine tests are performed to test for methadone, normally the test comes positive for 10-14 days or simply 2 weeks.
Weed or marijuana smoker may test positive for about 2 weeks, in usual urine tests. But the situation depends upon person to person, in some cases test may come positive even after long time also.
Best in your situation is quit both drugs by gradual tapering, do regular exercises, drink plenty of fluid only then you will test negative.