Characteristics and comorbidities of patients on opioid agonist therapy in Switzerland: A descriptive analysis of the nationwide SAMMSU cohort.

Background Opioid agonist therapy (OAT) is the gold standard of treatment for opioid dependence and a cornerstone of Swiss drug policy. The Swiss Association for the Medical Management in Substance Users (SAMMSU) cohort was established to monitor health trends and improve care for OAT patients across Switzerland.Methods Baseline and follow-up data collected from eight centres between 2014 and 2024 were analysed descriptively, including demographic and psychosocial characteristics, substance use history, prescribed OAT, co-medications, and somatic and psychiatric comorbidities.Results During the study, the SAMMSU cohort included 1 502 participants. Median individual age at registration was 44.3 years, rising to a cohort median of 50.9 years by the end of 2024; 75.7 % of participants were male. Lifetime heroin use was reported by 97.2 %, with 73.2 % having a history of intravenous drug use. Ongoing illicit and intravenous drug use declined over time, while prescribed OAT shifted from methadone to long-acting morphine and diacetylmorphine. The most prevalent lifetime somatic comorbidities were hepatitis C (56.5 %), (pre)hypertension (18.6 %), musculoskeletal disorders (13.8 %), and needle abscesses (13.7 %). Psychiatric disorders - primarily affective (34.8 %), personality (23.2 %), and anxiety disorders (18.0 %) - contributed to multimorbidity and a high prevalence of polypharmacy (49.2 %). There were 120 deaths, mainly from malignancy, overdose, and liver failure, with a median age at death of 51.6 years.Conclusion SAMMSU cohort trends corroborate the effectiveness of OAT in reducing illicit drug use and underscore the need for OAT services to evolve from an addiction-focused model to comprehensive chronic care for an ageing and highly vulnerable population.