Scoping Review Finds Only Lab Evidence Behind High-Dose Naloxone and Nalmefene

A scoping review published in Harm Reduction Journal has found that the clinical evidence base for two newer opioid overdose reversal strategies—high-dose naloxone formulations and the long-acting antagonist nalmefene—consists almost entirely of controlled pharmacokinetic studies in healthy volunteers, leaving a major gap in knowledge about how these drugs perform in real overdose situations.

The review, led by Brayden Kameg and colleagues, was conducted against a backdrop of sharply rising opioid overdose deaths in the United States over the past two decades, driven largely by the spread of illicitly manufactured fentanyls. Concerns that standard naloxone doses might be insufficient to reverse overdoses involving these potent synthetic opioids prompted the US Food and Drug Administration to approve high-dose naloxone products and the longer-acting antagonist nalmefene. Until now, however, synthesized evidence on these agents had been lacking.

To fill that gap, the researchers followed PRISMA-ScR guidelines and searched PubMed, Ovid Medline, and APA PsycINFO in September 2025. They included studies published since 2015 that evaluated intramuscular or intranasal high-dose naloxone at doses above 4 mg, or nalmefene, for overdose reversal. Intravenous formulations and preclinical studies were excluded. Two independent reviewers screened titles, abstracts, and full texts, with a third reviewer resolving disagreements.

Ten studies met the inclusion criteria: five examining high-dose naloxone and five examining nalmefene. Notably, all ten were pharmacokinetic or pharmacodynamic studies, and all but one were conducted with healthy volunteers rather than people experiencing overdoses. This study design profile is central to the review’s conclusions about the maturity of the evidence base.

For high-dose naloxone, the reviewed studies demonstrated dose-proportional increases in maximum serum concentration, meaning that higher doses produced correspondingly higher drug levels in the blood. Absorption rates were comparable to those of standard naloxone doses, suggesting the concentrated formulations do not act faster despite delivering more antagonist.

Nalmefene showed a different profile. Across the reviewed studies, it exhibited a longer terminal half-life than naloxone, meaning it remains active in the body for a longer period, and it produced a faster restoration of respiratory drive—the recovery of breathing that is the primary goal of overdose reversal.

On safety, both agents were generally well tolerated in the reviewed research, with no serious adverse events reported across the studies. The authors caution, however, that this tolerability record comes from controlled settings with healthy participants, not from chaotic real-world overdose responses involving fentanyl-contaminated drug supplies and people with varying opioid tolerance.

That limitation shapes the review’s central conclusion: current evidence on high-dose naloxone and nalmefene is confined to controlled pharmacokinetic investigations with limited generalizability to real-world overdose settings. The authors state that robust clinical outcome studies in community populations are urgently needed to guide evidence-based overdose response strategies.

For clinicians, public health planners, and people who use drugs and their families, the practical message is one of evidentiary caution rather than endorsement or rejection. High-dose naloxone and nalmefene are backed by mechanistic data showing predictable drug exposure, longer action for nalmefene, and no serious adverse events in trials so far—but the review’s authors emphasize that decisions about overdose response strategies should await clinical outcome research conducted in the communities where these products will actually be used.

Subject of Research: A scoping review of pharmacokinetic and pharmacodynamic evidence on high-dose naloxone and nalmefene for opioid overdose reversal

Article Title: Opioid antagonists for overdose reversal in the era of synthetic opioids: a scoping review of high-dose naloxone and nalmefene

Article References: Kameg, B., Matthews, E., Lindsay, D., & Manzour, R. (2026). Opioid antagonists for overdose reversal in the era of synthetic opioids: a scoping review of high-dose naloxone and nalmefene. Harm Reduction Journal. https://doi.org/10.1186/s12954-026-01541-8

Image Credits: AI Generated

DOI: 10.1186/s12954-026-01541-8

Keywords: naloxone, nalmefene, opioid overdose, fentanyl, scoping review, harm reduction, pharmacokinetics, opioid antagonists, drug safety, synthetic opioids, Opioid, antagonists