It is popularly believed that hair analysis can give an indication of the level of drug use. So an individual can be graded as a heavy user or a light user.
This is a dangerous over simplification.
There are many factors that affect levels of drugs and metabolites in hair. For example, differences in metabolism, purity and frequency of drug consumed as well as colour of hair and the effect of cosmetic hair treatment. Anything that damages the outer layers of hair, such as bleaches and dyes will reduce levels of drug in hair. When considering results of hair testing for drugs do think about what the donor may have been doing with their hair.
The main benefit of the use of hair samples in drug testing is the longer window of detection. That makes getting a false negative result very unlikely. In an individual one may assume that high levels detected in a hair sample reflect high use, however, this assumption can be misleading. Hair colour can markedly affect the level of drug taken up in hair strands. So a couple following the same pattern of drug use, including dose, can show remarkably different levels of hair drug levels if one is fair- haired and the other dark haired. The sensitivity of the testing will ensure that both are detected but the levels found could mislead investigators to believe one of the couple was using more drug than the other.
The levels of drugs in hair are best used as a guide to the changes in drug use in an individual. It is not possible to accurately compare data from hair analysis among individuals or determine the dose used.
However, it is possible to deduce that the same individual has used larger or smaller doses over several months, through the analysis of successive segments of hair.
The levels of drugs detected in hair are currently best used as a guide to changes of use in the individual when sectional analysis is performed or two different periods are compared in the same individual. This attribute can be used to monitor drug use patterns, demonstrating increasing or decreasing doses being used by the same individual over longer time periods:
Hair testing for drugs is very good for detecting use. It can give indications to changing levels of use over time that other samples cannot. It is a tool that can help when drugs are an issue, but as with any tool, needs to be used with skill.