The hair follicle drug test is a method used to detect drugs in the body by analyzing hair strands. When a drug, like marijuana or cocaine, is introduced into the body, it is metabolized and goes into the blood stream. Because hair gets its nourishment from the blood, the substance is transferred to the hair root and eventually finds its way to the hair strand which grows out of the root.

First, about 90 – 120 hair strands are collected from any part of the body. The hair strands may be from the head, armpit, chest or the pubis. Because hair thickness varies, it is necessary to test enough samples in order to arrive at an accurate result. Hair grows at an average of 0.5  inches a month, so strands of approximately 1.5 inches are used to determine the length of time a drug has been used. A hair strand of 0.5 inches will show whether or not there has been drug usage over the past 30 days while a 1.5 inch strand will extend this to 90 days. It takes about 4 – 5 days after a drug has been used for it to appear in a hair strand. Hair from other parts of the body grow at a slower rate and are not reliable indicators for a time frame of drug usage. The shortest time period of drug usage that can be tested is two weeks, using hair strands that are 0.25 inches long. If hair from parts of the body below the neck is used, the time frame increases to one year since body hair is usually replaced over a period of one year.

The hair follicle drug test analyzes substances inside the hair shaft, so external substances like residue from shampoos, gels and other styling aids will not influence the results of the test. Nevertheless, hair strands to be tested are cleaned to further minimize any risk of external elements compromising the test results. Hair color likewise has no effect on the results of a hair follicle drug test. Hair color is a function of melanin in the hair and does not have any influence on substances found in the hair strand.

Accuracy of the hair follicle drug test has been shown to be superior to that of the commonly-used urine drug test. The main advantages of the hair follicle drug test are a longer time frame and less susceptibility to tampering with the test sample. In addition to these, there is the convenience of the hair follicle drug test in securing the test sample and the inability to evade the testing procedure.