Will a hair drug test work if my hair is bleached or dyed?

Bleaching or dying of your hair will not prevent a positive drug test result, nor will it cause a false positive result.

Because hair drug testing detects molecules of the drug or drug metabolites in the hair shaft itself, external treatment of the hair will not alter the results. When a hair sample is tested for drug use, it is first washed to remove all external contaminants. The hair is then processed to extract the drugs and drug metabolites present in the hair shaft. These drugs and metabolites entered your hair through your bloodstream below the surface of your scalp or skin. They are not affected by dyes or bleached added to the hair once it emerges from your scalp. The substances obtained by the extraction process are then analyzed for the presence of drugs.

Some studies have indicated that cosmetic treatments such as bleach, relaxers, permanents, or dyes may lower the detectable concentration of selected drugs. However, these reductions remain within the standard range of uncertainty for gas chromatography-mass spectrometry (GC/MS) confirmation tests. These same studies indicate that cosmetic hair treatments had no impact on negative test results.