What Is A Hair Drug Test? In hair drug testing, also called hair drug screening, a person’s hair strand is used to find out which drugs were used by a person during the past few months. A small sample of hair is used, so the drug test identifies and measures all the molecules that are present in your bloodstream during the past 90-180 days. This type of drug test can tell the metabolic use of the drug that was produced after the drug was consumed by the human body. How Long Does Marijuana /Weed Stay In Your Hair?