Last updated: June 3rd, 2016
Hair drug testing will find traces of marijuana use much farther back in time than urine drug testing. In fact, when you ingest cannabis (or any drugs), its compounds make their way into the hair follicle through the blood stream, and then into the strands themselves, where they stay for quite some time. Hair grows at a rate of approximately .5 inches per month. So, in theory, if your hair was 18 inches long, and you tested the ends of the strands, then you could find drugs consumed more than 3 years ago…
But, don’t panic. No employer tests the ends of your hair (except the police, who can do whatever they wish). There is only one standard for this test in the workplace – 1.5 inches of hair, cut from the root. The non-root end is then discarded. In fact, we recommend that you watch the technician perform the test and ask them to cut off and discard the length beyond the first 1.5 inches. This ensures that the 1.5 inches equates to approximately 3 months (give or take) of history.
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