Indelibleface said:
Shave your head! :D
There's nothing you can do, as far as I know. I don't know enough about hair follicle tests to give you a proper answer. I'm aware of some hair products that claim to mask the detection schemes, but even if they did, I'm sure they have a way of detecting the "mask".
Haha, only if this person can shave their hair folicules.....otherwise shaving their head will do no good.
I will share a true story: approx. 4 years ago, a friend had to take a hair test. This person is/was a heavy pot smoker. We just *knew* they'd flunk the test, even with the "Special Shampoo" that this person bought at the local 'head shop'.
Since this person's living arrangements didn't exactly allow them to come in the house with "gonna wash this pot right out of my hair" shampoo, I agreed to let them come to my house. (And, I admit, I was curious!)
This person, in their desperate attempt to get this job, sat in my living room with the "special shampoo" on their head for FOUR HOURS. I advised against leaving it on for that long (couldn't be good for the scalp), but they were adamant in their decision to leave it on their head till it formed into a hard, dry, shampooed mess.
To make a long story short, this person passed their hair test. I don't know if it was the shampoo, the ridiculous amounts of water this person drank in order to *attempt* to flush their system, or what. All I know is they passed the test, still works for the same place, and it worked. (no, I don't remember the name or brand of the shampoo, and I doubt this person does either, but could try to find out if needed)
So who really knows?

I am guessing it would depend on the type of testing the lab will carry out. ect. At the time my friend had their hair test done, they were told it would show any useage in the past three months.
Maybe you should ask this question over in Drug Testing Q&A or Health Q&A?
Good luck!