The existence of drugs in the hair is based on a simple principle. Drugs are ingested into the body then flow into a person's bloodstream which feeds developing hair follicles. Therefore, small amounts of the target drug or drug metabolite collect in the hair follicle and become trapped in the center of the hair shaft as it grows out from the hair follicle. Average growth rates for human hair are roughly one-half inch per month. By testing for the drugs at different points in the hair shaft center on a piece of hair, a rather precise approximation can then be made as to how long a particular drug has been used.
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