A drug test involves analyzing a biological sample, such as urine, blood, or hair, to detect the presence of legal or illegal substances. Drug tests are used in various settings, with pre-employment screening being one of the most common applications. The substances tested can include legal ones like alcohol and tobacco, over-the-counter and prescription medications, as well as illegal drugs.

Commonly tested substances include amphetamines, cocaine, marijuana, opioids, and PCP. It's important to note that a single drug test cannot determine the frequency or intensity of substance use, which makes it difficult to distinguish between casual use and substance use disorders.

Because your body metabolizes different drugs at varying rates, the detection window for each substance can be highly specific, varying significantly from one drug to another. Urine drug testing (UDT) is the most frequently used method for detecting drugs, but hair testing is also a reliable option for detecting longer-term use.