Hair follicle testing, or what most call a hair drug test, detects illicit drugs, alcohol, or date rape drugs in a person's system. The test can identify substances like marijuana, cocaine, methamphetamine, ecstasy, phencyclidine (PCP), and opiates such as morphine.
The accuracy of the results is high, covering up to 90 days of drug use, and the sample collection process is easy to observe, minimizing the possibility of sample tampering, which is common with urine testing.
Hair samples about an inch and a half long are collected, representing three months of growth. The drug concentration in the hair increases as it grows, with the drug closest to the scalp indicating recent drug use. A positive test result usually requires at least three instances of drug use. This method is reliable since there is no way to manipulate the sample once it has been taken.