I've taken a couple of them before, the normal time they test for is 90 days (although technically the drugs would show up even longer)...they didn't take any hair from my head so they took it from my chest.
If you don't have ANY hair, they'll automatically fail you (or at least some places do) kinda like how some places will fail you for a diluted urine test or if you can't pee.
Basically...it's a druggies worst nightmare to be told they have to pass a hair test.
-edit- saw that you said it's been a couple months, then you should be good to go! If it really hasn't been a few months, there's a couple of techniques you can find around the internet if you have a few weeks to prepare for it...but the pass rate can still be pretty low.
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