If you have marijuana in your hair the simplest solution would be shower, if you're talking about drug testing keep reading...
If you haven't smoked recently it should be gone with 2-3 days. Constant users are detectable up to several weeks, longer than any other illegal substance is detectable.
Your best defense:
1) Shave your head. No hair, no marijuana!
2) Switch to harder drugs.
3) Wear a wig made of REAL HUMAN HAIR. Nothing fake or polyester, those tests sample the pH and they'll know something fishy is going on when they find out that they have a sample of your cat's/ferret's/barbie doll's hair.
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Get a very good Lice removing treatment (shampoo, kits, etc). Make sure you thoroughly wash your hair with it. Depending on how long you have to remove the substance from your hair, you might want to consider buying a Toxin hair removal kit. For example, there is a store in my town called Yo Yo Dyne, and they sell all sorts of detox to remove marijuana from your system ASAP. Another option to consider is dying your hair. Bleaching definitely works but if you have dark hair you can go a shade darker and it will still work. There is a way around everything.
Shave your head. Or, to be safe, wax or shave your whole body as more and more hair screenings are done with other bodily hair.
THC(active chemical in marijauana) is stored in the hair folicle. Because THC love to store its self in fat cells. And is fat soluble. Happy toke'n
shave your head, hair tests cannot be faked, you can try soaking the strand in an alcohol solution, but it is not a fool proof method
a razor. It's like the rings on a tree once its in the follicle it's there for good.
You cannot get rid of marijuana in your hair. However, after about 45 days it gets removed by itself.