Purpose <p>Hair testing for drugs has been used extensively in the forensic field since the 1990s, primarily in cases involving abused drugs such as methamphetamine and cocaine. Since the 2010s, its scope has expanded to include the detection of single dose of hypnotics, aiding in the investigation of serious crimes. This review presents essential knowledge for hair testing and the currently recommended analytical procedures and forensic applications.</p> Methods <p>A review of literature from the 1990s to the 2020s was conducted, focusing on analytical methods for detecting drugs in hair, drug concentrations in hair, and drug incorporation pathways.</p> Results <p>The characteristics of hair as a biological specimen include a longer detection window than other matrices such as urine and blood, as ingested drugs remain stable in hair over time. Significant differences in drug concentrations in hair are observed among substances, with several hypnotics, such as triazolam, having extremely low concentrations. Drugs are incorporated into hair primarily through two main pathways (the hair bulb and the upper dermis zone), with the dominant pathway depending on the drug’s properties. In addition, hair dyeing and subsequent exposure to aqueous environments (e.g., daily hair washing) can significantly influence drug concentrations and their distribution patterns (concentration and hair region). These factors must be carefully considered in hair testing.</p> Conclusions <p>Hair testing is an effective means for proving drug intake and estimating use history, particularly in cases where there is a delay in reporting the incident. The interpretation of results must account for various factors, such as the chemical structures of drugs, incorporation pathways, and hair dyeing.</p>