Self-reported cannabis use doesn't match hair test results

The growing prevalence of substance use and its associated health consequences highlights the need for reliable approaches to assess consumption patterns and validate self-reported data. This study, conducted at an international music festival in Portugal, aimed to characterise substance use by integrating objective toxicological findings with self-reported information. Quantitative hair analysis was combined with survey data from 249 participants recruited in 2022 and 2023. Hair samples were analysed by liquid chromatography-tandem mass spectrometry to detect psychoactive substances and metabolites. Self-reported use was assessed across multiple timeframes, from same-day consumption to use within the previous year. Alcohol (96%) and cannabinoids (90%) were the most frequently self-reported substances overall, based on lifetime self-reported use. Overall, 50% of participants tested positive for at least one compound in the analysed hair samples, with cocaine, MDMA and ketamine being the most commonly detected substances (24.5%, 24.1% and 22.9%, respectively). Some participants who denied consumption tested positive, particularly for MDMA and ketamine. Self-reported non-use was inversely associated with hair positivity for MDMA (OR = 0.25, 95% CI: 0.09-0.65) and ketamine (OR = 0.19, 95% CI: 0.09-0.40), compared with self-reported users. Discrepancies were also observed for cannabinoids, highlighting limitations of self-reported data. Strong correlations were identified between cocaine and benzoylecgonine, and between cannabis-related analytes (cannabidiol and THC), supporting the consistency of the toxicological results. Integrating self-reported data with objective biological measures improved data reliability, revealed polydrug use patterns and supported substance use monitoring in high-risk populations such as music festival attendees in this high-exposure festival setting.