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How far back does a hair drug test go?

A:

Quick Answer

A hair drug test goes back as far as three months because testing uses about 1 1/2 inches of hair, according to the Drug Testing Network. However, after using a drug, the hair containing the drug evidence does not grow out of the scalp for at least a week.

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Full Answer

For a hair test to come back as positive, there must have been more than one use of the drug, making it nearly impossible to test positive due to passive exposure, states the Drug Testing Network. Hair tests usually test up to 30 days of drug use, but only take one to three days to get a result.

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