I don't want to be -ve about it or anything, and nor do I have any experience yet of drug tests. I have been just doing some research for my upcoming test.

From this article, it is pretty clear what you can expect.

Unfortunately, nothing works to fool a hair test. They can only use 1.5" (They CAN use more, but cannibinoids break down in the hair a little after 90 days, so long hair is NOT a "timeline". It's just long hair). Shaving your head also won't work, they'll just find it somewhere else (Pubic, chest, back, anywhere really) and they will also make a note that your head was recently shaved. The cannibinoids are stored WITHIN the hair folicle, so those "cleansing" shampoos are total bullshit also. Like I said, save your money,