Discordance between self-reported and detected synthetic opioids reveals cychlorphine emergence among people who inject drugs in Estonia.

INTRODUCTION: Since 2002, Estonia has experienced a changing illicit synthetic opioid market, with the emergence of highly potent substances posing a substantial threat to public health. Over the transitional periods, people who inject drugs have faced high risks, often without knowing what substance they are using. This study assesses the agreement between self-reported and detected drug use and examines the presence of synthetic opioids in urine and how long they remain detectable. METHODS: Out of 350 individuals in a cross-sectional bio-behavioural survey among people who inject drugs in Kohtla-Järve, Estonia, 112 participants (32.0%) consented to a voluntary toxicology sub-study by providing a urine sample and completing a questionnaire on drug use within the past 7 days. Urine samples were tested using liquid chromatography-quadrupole time-of-flight mass spectrometry (LC-Q-TOF-MS). RESULTS: Self-reported use of synthetic opioid (fentanyl, nitazenes, "heavy") was common (33.9%), whereas toxicological confirmation was rare. Half of these individuals had cychlorphine detected (50%), indicating widespread misidentification of substances and suggesting substitution within the drug supply. Overall, cychlorphine was toxicologically detected in 22 cases (19.6%) among individuals with both reported and non-reported synthetic opioid use. Cychlorphine was detected in urine at a mean of 17.1 h after self-reported use, whereas no detection in urine occurred on average at 47.6 h after self-reported use. CONCLUSION: There is a substantial gap between self-reported and actual synthetic opioid use within a rapidly evolving opioid market. Emergence of previously unrecognised substances such as cychlorphine place people who inject drugs at heightened risk of overdose.