http://en.wikipedia.org/wiki/Drug_test#Hair_drug_screen
Hair drug screen
Hair follicle testing is quite accurate and can go back 6 months or longer, showing any controlled substances used in a sort of timeline. As hair grows out, any drugs used are encased in the hair shaft, so the longer the hair, the longer back in the individual's drug history the lab can detect. Most legitimate testing facilities, however, only use hair within about 3-5 cm of the scalp, and discard the rest. This limits the detection history to about 90 days, depending upon the rate of growth of the individual's hair. Some people attempt to circumvent this through shaving their heads. In the absence of the required amount of hair on the scalp, body hair can be used as an acceptable substitute. Additionally, for pre-employment hair testing, the inability to obtain a sample may be grounds for not hiring the individual.
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