Listen, I think there is some confusion here, and it may well be mine (and agreeing with SWPK):
Let's look at a normal hair as it sits in the scalp:
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(https://forum.drugs-and-users.org/proxy.php?request=http%3A%2F%2Fwww.hairtx.com%2Ffiles%2F2014%2F03%2Fhair_anatomy.gif&hash=490f1c75596f2c576d2aab6bdaa08e499b37bc18)
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Now, for DNA testing (to prove paternity, or to match the DNA to a semen sample, say from a rape, etc...), they need the FOLLICLE. It contains cells, which contain nuclei, which holds your unique DNA sequence.
The follicle doesn't provide information regarding drug use, as far as I know, only DNA in the cells surrounding the hair. Also, they cannot just CUT the hair with scissors, they have to PLUCK it, to get the cellular material with it (little clump of white stuff at the base).
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Now for Drug Testing, they will look at the hair shaft itself as a Time Line of drug use, drugs used and the amount of drug(s) used. They don't need to pluck the hair, they don't need the follicle, just the hair shaft--so scissors will work.
Assume that hair grows about 1cm per month, and they take a hair that is 12cm long, they could, probably figure-out:
1- What drugs were used,
2- When those drugs were used,
3- How much drugs were used (by measuring the amount of metabolites in the hair shaft).
See the image below--it gives you an idea: almost like a tape recording: if drugs metabolites were found 4cm from the root (follicle) means that drugs were used ~4 months ago (or whatever the growth rate is determined to be).
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(https://www.ucl.ac.uk/news/news-articles/1114/300hair.jpg)
As far as I know, those drug metabolites are PART OF the hair itself. They're aren't just some greasy coating on the hair, like paint on a wall, but part of bricks that make up the wall.
One would have to destroy the hair itself to remove the metabolites.
Again, I could be 100% wrong. It would be difficult to beat this test, if it is accurate. Shaving (Nairing) all of the hair off of your body would work, I think--kind of like not having any urine for them to test, you have no hair for them to test.
I was told this is one reason why PoPo keep their hair short--does that make sense? Maybe if you think about them playing in the evidence locker, or only 75% of a drug seizure getting turned in, it makes sense.