Det er visst en smule mer komplisert enn som så:
[Hair tests] Can sometimes be used to determine when use occured and if it has been discontinued. Drugs, such as opiates (codeine, morphine, heroin) lay down on the hair shaft very tightly and are shown not to migrate along the shaft, thus, if a long segment of hair is available one can draw some "relative" conclusions about when the use occurred. However cocaine, although very easy to detect, is able to migrate along the shaft; making it very difficult to determine when the drug was used and for how long.
De nevner ikke noe om hvordan cannabismetabolitter oppfører seg i hår.
Erowid nevner at ting kan spores opp til 90 dager tilbake i håret:
http://www.erowid.org/psychoactives/...shtml#duration så det kan hende at etter ca 90 dager så vil disse metabolittene brytes ned så mye at tester ikke kan påvise noe.