Unregulated Substance Abuse and Systemic Inflammation Markers: A Review

Aim: There is an urgent need for systematic and well-designed studies to clarify the role of systemic inflammatory parameters, especially the neutrophil–lymphocyte-ratio (NLR), in the pathophysiology and clinical management of unregulated substance addiction. This review aims to synthesize current evidence on the relationship between unregulated substance addiction and systemic inflammatory parameters, focusing specifically on the NLR as a potential biomarker. Methods: To ensure a transparent approach in the collection of evidence, this review was carried out following the recommendations of the PRISMA 2020 guidelines and registered in PROSPERO (CRD420251151136). We searched the PubMed and Scopus databases in July2025 using combinations of MeSH terms and keywords related to unregulated substance use and inflammatory biomarkers. The strategy included terms such as “cocaine,” “cannabis,” “opioids,” “heroin,” “fentanyl,” “methadone,” “buprenorphine” “nitazene”, “MDMA”, and “methamphetamine,” combined with “neutrophil-to-lymphocyte ratio.” Filters were applied to limit results to human studies published between 2015 and 2025 in English. The methodological quality of the studies included was assessed using the STROBE 22-item checklist. Results: Fifteen studies were included in this review. Methamphetamine and opioid users showed higher NLR and MLR values. For cocaine abuse, although the evidence is limited to a single population-based study, a significant increase in NLR was reported. Controversial results were observed for cannabis use. Conclusions: Systemic inflammation markers are related to unregulated substance abuse disorders; however, the sparse available evidence encourages the need for well-designed large, prospective clinical trials.