Prieto, J, Subirá, M L, Castilla, A et al. · Scandinavian journal of immunology · 1989 · DOI
Researchers examined immune cells called monocytes in 35 ME/CFS patients and 25 healthy people. They found that most ME/CFS patients had monocytes that weren't working properly, but these cells improved dramatically when treated with naloxone, a drug that blocks opioid effects. This suggests that the body's natural opioid-like chemicals may be overactive in ME/CFS and could be damaging immune function.
This study proposes a novel mechanism for ME/CFS pathology—dysregulated endogenous opioid activity—that could explain immune dysfunction central to the disease. If endogenous opioids are indeed pathogenic, this opens therapeutic avenues such as opioid antagonists, potentially offering a targeted treatment strategy rather than symptom management alone.
This study does not prove that elevated opioids directly cause ME/CFS, only that opioid-mediated dysfunction exists in monocytes from most patients studied. It does not measure actual endogenous opioid levels in the blood or tissues, so the source and magnitude of opioid excess remains uncharacterized. Cross-sectional design prevents determining whether opioid dysfunction precedes disease onset or results from it.
About the PEM badge: “PEM required” means post-exertional malaise was an explicit required diagnostic criterion for participant inclusion in this study — not that PEM was studied, observed, or discussed. Studies using criteria that do not require PEM (e.g. Fukuda, Oxford) are tagged “PEM not required”. How the atlas works →
The first block is for the primary paper and is the citation you should use in research work. The atlas-snapshot line only applies if you are specifically referring to this atlas’s reading of the paper on the date shown.
Primary citation
Prieto, J, Subirá, M L, Castilla, A, & Serrano, M (1989). Naloxone-reversible monocyte dysfunction in patients with chronic fatigue syndrome.. Scandinavian journal of immunology. https://doi.org/10.1111/j.1365-3083.1989.tb01183.x
BibTeX
@article{mecfsatlas-prieto-1989-naloxone-reversible,
author = {Prieto, J and Subirá, M L and Castilla, A and Serrano, M},
title = {Naloxone-reversible monocyte dysfunction in patients with chronic fatigue syndrome.},
journal = {Scandinavian journal of immunology},
year = {1989},
doi = {10.1111/j.1365-3083.1989.tb01183.x},
note = {PubMed: 2526966},
url = {https://www.mecfsatlas.com/evidence/prieto-1989-naloxone-reversible},
}Atlas snapshot reference
ME/CFS Atlas. Generator v1 / Scanner v1.4 / policy v0.1. Accessed 2026-05-30. https://www.mecfsatlas.com/evidence/prieto-1989-naloxone-reversible
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