No shampoo is going to clean your hair of evidence of cannabis use, so don't waste your money. You need about 90 days of cannabis-free history in order to test clean on hair testing. I'm amazed they didn't bounce you out of the testing facility after that story about having been in the car with other smokers five months ago. Evidence of cannabis stays in your hair for up to a year, but hair-testing labs only take the most recent 90 days of growth, the part of the hair nearest your scalp, for testing, which is why the 90-day detection clearance period is pretty universal. You can read more at the link below.

Erowid Drug Testing Vaults : The Basics