Hair drug tests detect drug use by analyzing a small sample of hair, typically from the scalp. This type of testing can identify a wide range of substances, including marijuana, cocaine, opiates, and amphetamines, over a much longer detection window, usually up to 90 days after use. The process involves cutting a small amount of hair close to the scalp and sending it to a laboratory for analysis, where the hair's drug concentration is measured.

 

One advantage of hair drug testing is its ability to provide a historical record of substance use, which can help in cases like employment screenings or legal investigations. However, it's important to note that hair drug tests may be less effective at detecting very recent use and may be influenced by factors such as hair color and cosmetic treatments.