Inconsistent evidence on the impact of lay worker-delivered psychosocial interventions on substance use, mental health and quality of life among people who use illicit drugs outside healthcare settings: a systematic review

People who use illicit drugs often avoid formal healthcare due to stigma and legal concerns. Lay worker-delivered interventions outside clinical settings offer an accessible alternative. This review evaluated the effectiveness of such interventions on substance use, mental health, and quality of life and explored the patterns across intervention characteristics, target populations and effectiveness. We included original studies with trial and pre-post designs. We searched PubMed, Cochrane CENTRAL, Embase, APA PsycINFO, trial registries and conducted citation searches. Title and abstract screening was assisted by machine learning; full-text screening, data extraction and risk of bias assessment were conducted independently by two reviewers. Findings were synthesized using a narrative approach. Thirteen studies met inclusion criteria: eight targeting people who use drugs in general, two focusing on women, two on men who have sex with men and one on chronic pain patients. Most studies were US-based with one study from a low-and-middle-income country. Risk of bias was low in one study, moderate in seven and high in five. Outcomes were synthesised based on the eight studies with low or moderate risk of bias (six randomised controlled trials and two pre-post studies). Substance use outcomes improved significantly in most studies; mental health outcomes also improved but rarely reached statistical significance. However, evidence on the additional benefits of lay worker-delivered components was inconsistent, with three trials reporting favourable effects (two statistically significant, p = 0.015 and p = 0.03; the third with significance not specified), and three trials reporting no significant differences. Trials with positive findings primarily focused on harm reduction and employed brief, structured interventions. The small number of studies and their methodological limitations warrant cautious interpretation. Lay worker-delivered psychosocial interventions outside healthcare settings may offer an accessible approach for people who use illicit drugs. However, current evidence does not demonstrate consistent additional benefits compared with established interventions, and no clear patterns in effectiveness were identified across populations or intervention characteristics. These findings highlight the need for adequately powered trials incorporating strategies to improve retention and adherence, and implementation-focused research to evaluate the delivery of evidence-based interventions by lay workers.