Syringe residue analysis reveals polydrug use patterns and emerging trends in Barcelona

Background

Syringe residue analysis is an objective method to monitor substances used by people who inject drugs (PWID), introduced by the ESCAPE network in 2017. Within the SYDRES project (2023–2025), it was applied in Barcelona to characterize consumption patterns, detect emerging substances, and complement surveillance systems

Methods

A total of 304 used syringes were collected from harm reduction services (supervised consumption rooms and outreach programs). Samples were analyzed using liquid chromatography–high-resolution mass spectrometry following ESCAPE protocols. A spectral library (>200 compounds) enabled identification of drugs, metabolites, and adulterants. Sociodemographic (self-reported gender) and contextual data (including chemsex) were included

Results

Cocaine was the most frequently detected substance (88%), often combined with heroin and other opioids, indicating widespread polydrug use. Around two-thirds of samples contained ≥3 substances. Gender differences were observed: samples from women showed higher prevalence of amphetamines and more complex combinations, while those from men more often contained cocaine–opioid combinations (p = 0.017). In chemsex-related samples, stimulants predominated, especially amphetamines and synthetic cathinones. Temporal trends suggested a decrease in opioid-based combinations and an increase in stimulant-related patterns in 2025. Adulterants such as piracetam and dextromethorphan were also identified

Conclusions

Syringe residue analysis confirms the central role of cocaine and highlights polydrug use as a key feature of injecting practices in Barcelona. It enables early detection of emerging substances and evolving patterns, including chemsex-related use. Incorporating gender perspectives and strengthening harm reduction responses are essential. This approach is a valuable tool for public health surveillance

Key messages

• Syringe residue analysis detects emerging drugs and widespread polydrug use, strengthening rapid public health surveillance and early warning systems.

• Integrating syringe monitoring into harm reduction improves early alerts and supports gender-responsive responses.