Determination of the Optimal Cutoff Point for the ChemSex Inventory (CSI) to Assess Chemsex-Related Mental Health Risk in Men Who Have Sex with Men

Chemsex poses growing psychological and behavioural risks among men who have sex with men (MSM). Although the Chemsex Inventory (CSI) was developed to assess these risks, clinical cutoff points distinguishing recreational use from pathological patterns remain undefined. This study aimed to determine optimal CSI cutoff points to identify profiles of increased psychological and behavioral vulnerability associated with chemsex. A cross-sectional study was conducted in Spain between January and April 2023 with 563 MSM who reported chemsex use within the previous 12 months. Psychometric analyses included principal component analysis (PCA) and confirmatory factor analysis (CFA) of a second-order model with four dimensions: emotional instability, risk of psychosis, altered body perception, and suicide risk. Cutoff points were determined using the percentile-based P-score method. The CSI showed excellent internal reliability (Cronbach’s α = 0.88; ω = 0.88) and strong factorial validity (CFI = 0.985, TLI = 0.984, RMSEA = 0.05). The second-order model fit significantly better than the first-order alternative (χ²diff = 8.14; p = 0.017). High-risk cutoffs were: emotional instability ≥ 78.3, psychosis ≥ 51.4, body-perception disturbance ≥ 95.0, suicide risk ≥ 75.0, and global ≥ 66.4. About 5% of participants exceeded these thresholds, indicating elevated psychological vulnerability. CSI cutoff points provide a reliable and sensitive tool for early detection of mental-health risks related to chemsex. Incorporating the CSI into sexual-health, primary-care, and mental-health services may enhance early detection of at-risk individuals and support evidence-based prevention and harm-reduction strategies.