Routine Hair Testing Unmasks Hidden Synthetic Cannabinoid Use in Forensic Psychiatric Patients: A 10-Year Comparative Study in Two Bavarian Clinics

Background: Germany provides a worldwide almost unique legal framework for offenders with substance use disorders through § 64 of the German Criminal Code, mandating a two-year multimodal therapy including an in-house clinical treatment period followed by a reintegration phase with gradually reduced supervision. During this phase, lapses are often concealed, with synthetic cannabinoids (SCs) serving as a potential tool due to limited detection in routine screenings and heterogeneous monitoring practices across forensic psychiatric clinics. Methods: This study compared two forensic hospitals, Guenzburg and Kaufbeuren, over a ten-year period, from 2014 to 2024, to evaluate monitoring strategies. While Kaufbeuren applied a case-dependent testing approach, including unannounced urine screenings, Guenzburg introduced a stricter regime in 2019, combining mandatory hair analysis three months into reintegration with unannounced broad-spectrum screenings including SCs. Results: Among the 527 patients included in this study, significantly more tests were conducted in Guenzburg after 2022. The different approach between hair vs. urine analysis produced a seven-fold higher detection rate of SC use compared to Kaufbeuren. Across both clinics, however, SC-positive patients shared similar features. They were younger at first conviction, more frequently under substitution treatment, and more likely to have committed violent offenses. A history of violence quadrupled SC-positive odds, while time since leave as such increased odds by 0.1% per day. Conclusions: In conclusion, these results underscore the effectiveness of standardized long-term SC monitoring using hair analysis and the predictive role of a history of violence in the context of SC-relapse.