Hair drug testing is considered the most accurate drug testing procedure and can go back as far as 90 days. As hair grows out, any drugs or toxins are encased in the hair shaft, so the longer the hair, the further back in time the hair drug testing lab can go. Human hair grows an average of .5 inches/month, which corresponds to 30 days of possible drug testing. Hair drug test laboratories generally require 1.5 inches for testing. This means they can test for around 90 days of drug use. Some people attempt to avoid this by shaving their heads. This does not work. If you have no hair on your head, they will go elsewhere for the hair. Which of course is more painful.