The test they do on hair is a Gas Chromatography/Mass Spectrometry to find any metabolites of THC on your hair. They usually will take the hair sample and drop a few drops of solvent on the hair and collect what drips off your hair into a vial. That vial gets tested, and they look for heavy concentrations of those metabolites. One thing I would try is getting your own sample prepared ahead of time. Use 91% or higher rubbing alcohol and use an eyedropper to "rinse" your hair. The rubbing alcohol should dissolve those metabolites, because it's a great solvent for THC (think iso hash).

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D.I.whY? Because I CAN!