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. Collecting comparable samples for repeat testing is easier with hair than with urine. However, there are several reported problems with hair drug tests. The Federal government, which has set strict standards for urine testing, has not set standards for hair tests because it is not yet convinced of their accuracy. Another controversial aspect of hair drug testing is that it may yield racially-biased results.