Just doing a little research and figured this information could be helpful to some people. Despite what most drug testing companies say, cosmetic treatments do affect how much of the drugs is detected in your hair. Dying or, even better, bleaching your hair lowers the concentration of metabolites in your hair. It is different for different people and for different drugs, but it does lower it. Here’s a link to a journal article discussing it and some highlights. For THC this article is rather vague, because it only used one sample with bleached hair and that produced only a small decrease in THC and THC-COOH. But for the dyed hair samples there was a mean decrease of about 60% in THC-COOH concentration. The article is: C. Jurado, P. Kintz, M. Menéndez, M. Repetto, Influence of the cosmetic treatment of hair on drug testing, International Journal of Legal Medicine, Volume 110, Issue 3, Apr 1997, Pages 159 - 163 And can be found here: http://springerlink.metapress.com/o...e&issn=0937-9827&volume=110&issue=3&spage=159 Abstract: "Treated and untreated portions from each lock of hair were then selected, separated and analysed by standard detection procedures for cocaine, opiates, cannabinoids and nicotine. In all cases the drug content in hair that had undergone cosmetic treatment decreased in comparison to untreated hair. The majority of the mean differences were in the range of 40%-60% (cocaine, benzoylecgonine, codeine, 6-acetylmorphine and THC-COOH). For morphine the mean difference was higher than 60%, and two cases (THC and nicotine) differed by approx. 30%. These differences depended not only on the type of cosmetic treatment, as bleaching produced higher decreases than dyeing, but also on the degree of hair damage i.e. the more damaged the hair, the larger the differences in the concentration levels of drugs."