Photodegradation of Drugs of Abuse in Hair

Hair analysis is a valuable tool in clinical and forensic toxicology to demonstrate drug exposure when cases of chronic intoxication, use, abuse, or single dose consumption need to be diagnosed in the context of facilitated crimes, withdrawal controls, doping controls, or workplace drug testing, with a large window from weeks to months/years for drug detection. However, scalp hair is exposed to sunlight and/or artificial light for many hours per day; hence, the action of light on hair could alter the content of drugs/illicit drugs and/or metabolites and the xenobiotics can gradually disappear from the hair shaft or be transformed into other compounds having a different structure from the parent molecule. Thus, light exposure should be considered as a potential confounder in studies investigating xenobiotics in hair giving rise to reduced drug concentrations or even false negative results. On the other hand, the formation of new photodegradation products could lead to the identification of new markers of abuse useful in forensic evaluations. Although the importance of the potentially detrimental effect of light on the exogenous molecules present in the hair shaft is being recognized, very few studies are actually present in the literature about the photodecomposition of illicit drugs in this valuable biological specimen.