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. Preliminary research shows that even treating hair with a variety of strong compounds will not completely eliminate traces of illicit drugs. Compared with urine samples, it presents fewer risks of disease trasmission. Some practical difficulties may occur in collecting specimens from individuals with short or no head hair.
Metabolites are the biochemical products of the breakdow of drugs within the body.
For cocaine, both urine testing and hair testing detect the drug metabolites rather than the illegal drug itself. Once a drug metabolite is embedded in the hair shaft, a process which appears to occur while the hair is being formed in its follicle, the metabolite is very nearly permanent. As the hair shaft grows, it forms a longitudinal record of the compounds it has absorbed, including drugs of abuse. Drug metabolites appear in detectable levels in hair approximately a week after ingestion. Hair grows at an average rate of about half an inch every 30 days.