Will you fail a hair test I you smoke marijuana once three weeks before the test?

Depends on how long you've smoked before that. Typically, unless you were an abnormally heavy smoker, I'd say no. A good tip is to work out and drink plenty of fluids a week before the drug test (and then seize physical activity the days before), this allegedly can "flush" the THC which is stored in your fat out of your system.

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