As a result of this, any drug or drug metabolite will be deposited in the hair when the blood reaches it, and will be stored there until the hair is cut off.  Hair normally grows at something like half an inch per month, so by testing hair at different lengths a laboratory can confirm when and what a subject has taken.

Usually only the first one and a half inches of hair are tested, which is three months growth.  This is a standard hair drug test.  If a longer record is needed, then more hair will be needed and tested.

Hair drug testing is so accurate that many courts and legal entities accept is as evidence.  The drug compounds remain stable once they are in the hair and can provide evidence of drug taking for months on end.  As most of the tests also test for metabolites, the test is conclusive of the subject having taken the drugs, not just being in the vicinity of it.

Metabolites are chemicals formed once the drug has been ingested, then metabolized by the body.  This is definitive evidence that the drug has been taken.

The downside to hair drug testing is that it needs around 50 hairs to be completely accurate.  Not many people will voluntarily give up their hair like that.  The least obtrusive place to obtain them is the crown.  There a sample, cut as close to the scalp as possible is taken and tested.  If a subject has no hair, then body hair can also be used.  Using body hair needs different calculations as its rate of growth is much slower than head hair.

There are over the counter shampoos that purport to wash off the chemicals in the hair.  Also there is a school of thought that hair dyes and treatments also lessen the effectiveness of hair drug testing.  These theories are just that.  A hair drug test analyzes the internal structure of the hair.  Any amount of treatment or washing isn’t going to affect the inside of it.

Hair testing is less open to abuse as the entire procedure can be performed and controlled by the agency doing the tests.  Unlike urine testing where substitution or contamination can occur, with hair testing it cannot.  As long as the correct procedures are followed when the sample is collected the test should be completely viable.

Because of the accuracy of its results, hair testing is gaining in popularity.  Once the barrier of the subjects unwillingness to provide the sample is removed, it is one of the most accurate, and non-invasive drug testing methods available.