Reaching the hair
For a drug to get into the hair from inside, it first has to enter a person’s bloodstream. This happens when a drug is injected into a vein, or when it’s smoked, snorted, or swallowed.
Once in the bloodstream, the drug travels to every part of the body, including the hair. Some drugs, such as morphine, cocaine, nicotine, and amphetamines, then attach to the melanin (which gives hair its color) in each strand of hair.
As far as drugs entering the hair from the outside, when drugs are smoked, some of the smoke or vapor enters the air. Some drugs, such as nicotine and heroin, then can easily enter a person’s hair and attach to melanin. Even with a good shampoo, some of the drug will stay inside the hair.
Testing hair for drugs
No matter how a drug reaches the hair, researchers have developed very sensitive chemical techniques to detect drugs there. If it got to the hair from inside the body, tests can even tell when the drug was used. A drug found in the hair root was used very recently; if it’s closer to the end of the hair, the drug may have been used months before.
There’s a catch, though: If a person’s hair tests positive for a drug, how do the testers know if the drug came from the person’s environment (from secondhand smoke, for instance) or from drug use? As testing techniques get more precise, it will become easier to answer that question. And, that’s why urine and blood are also often tested for drugs.