Traces remain in the hair, which show how long the drugs have been used. Since hair grows at the rate of about a half-inch a month, the test uses the inch and a half closest to the scalp to detect drug use for the last 90 days. The Department of Justice lists the following advantages of testing hair rather than urine for illicit drugs. Hair greatly expands the time window for the detection of an illicit drug. Urinalysis of a single specimen generally can detect the presence of drugs for a period of several days to a week or two, depending on the drug.
Hair analysis can detect drug use for several months or more, depending on the length of the hair.
Brief periods of abstinence from drugs will not significantly alter the outcome of hair analysis. Contaminating or altering a sample to distort or manipulate test results is much more difficult with hair than with urine. Preliminary research shows that even treating hair with a variety of strong compounds will not completely eliminate traces of illicit drugs. Hair is relatively inert, easy to handle, and requires no special storage facilities or conditions. Compared with urine samples, it presents fewer risks of disease transmission.