The humble mucus drug N-acetylcysteine may be medicine’s most versatile repurposing candidate

N-acetylcysteine, a cheap and long-available drug best known for loosening mucus in patients with chronic lung disease and for reversing paracetamol overdoses in emergency rooms, may be one of the most extensively studied repurposing candidates in modern medicine. A new scoping review of reviews, published in PLOS One, has mapped the entire landscape of evidence surrounding this unassuming molecule and found that it has now been examined across seventeen distinct clinical domains, from psychiatry to reproductive medicine. Yet the analysis also delivers a sobering warning: despite decades of research and hundreds of published syntheses, the quality of that evidence is often critically low, and the drug still lacks the consistent, high-certainty support needed to earn a place in clinical guidelines for most of its proposed new uses.

The study, led by Mohammed Khaled Al Alaili, took an unusually rigorous approach for an overview of this kind. The researchers systematically searched five databases to identify reviews of N-acetylcysteine that met predetermined inclusion and exclusion criteria, ultimately retrieving 140 separate reviews spanning the full breadth of clinical medicine. Rather than simply cataloguing these publications, the team extracted comprehensive data to map the evidence, and then applied two validated appraisal tools to assess methodological quality and risk of bias. This allowed them to stratify the evidence by how much confidence clinicians and researchers should actually place in it, a step that many broader reviews skip entirely.

One of the most striking findings concerns the hierarchy of evidence itself. Of the 140 reviews retrieved, only 63 were systematic reviews with meta-analysis, the design widely regarded as the highest level of synthesized evidence in medicine. The remainder were narrative or other less structured reviews, which are more vulnerable to selective citation and author bias. Among the 63 top-tier syntheses, roughly half supported repurposing N-acetylcysteine for new indications, a proportion that sounds encouraging until it is set against the quality appraisals. The critical appraisal revealed a high prevalence of critically low-quality systematic reviews, even as a notable proportion of reviews showed low risk of bias, a mismatch that underscores how uneven the field really is.

To quantify redundancy, the researchers calculated the corrected covered area, a metric designed to detect when multiple reviews cover the same primary studies, inflating the apparent volume of evidence without adding new information. The CCA analysis demonstrated substantial overlap and redundancy across domains, and particularly within subdomains. In practical terms, many of the 140 reviews were re-analyzing the same small pools of trials, a pattern the authors describe as possible research waste. This finding matters beyond N-acetylcysteine: it illustrates a systemic problem in evidence synthesis, where the incentive to publish reviews outpaces the production of new primary trials capable of settling clinical questions.

The biological case for repurposing N-acetylcysteine rests on its remarkable pleiotropy. Most frequently reported in the literature are its antioxidant properties: the drug replenishes glutathione, the cell’s principal endogenous antioxidant, thereby buffering the oxidative stress implicated in conditions ranging from neurodegeneration to chronic kidney disease. Alongside this, the review catalogued well-supported anti-inflammatory effects and an increasingly studied role in modulating glutamatergic signaling, the neurotransmitter system implicated in compulsive behavior, addiction, and several psychiatric disorders. Beyond these three headline mechanisms, the authors note that several additional mechanisms have been proposed, reflecting the molecule’s involvement in fundamental cellular pathways rather than any single disease-specific target.

The geographic and temporal patterns in the literature are equally revealing. The number of N-acetylcysteine reviews has increased over time, mirroring the broader surge of interest in drug repurposing and oxidative stress as therapeutic frontiers. Most of these reviews originated from China and the United States, a concentration that may shape which questions get asked and which patient populations the evidence reflects. Drug repurposing has become one of the most attractive strategies in pharmaceutical research because established agents come with known safety profiles, existing manufacturing capacity, and often low cost, and N-acetylcysteine exemplifies all three advantages, making the gap between research enthusiasm and clinical implementation all the more conspicuous.

From the mapped landscape, five clinical domains emerged as priorities for future exploration: psychiatry, reproductive medicine, nephrology, pulmonology, and toxicology. Each of these fields has plausible mechanistic rationale for benefit. In psychiatry, glutamatergic modulation offers a testable pathway for disorders of compulsion and mood. In reproductive medicine, oxidative stress is a recognized contributor to fertility problems and pregnancy complications. In nephrology and pulmonology, antioxidant and anti-inflammatory actions align with the pathophysiology of chronic organ injury, while toxicology already has one firmly established use, the treatment of paracetamol poisoning, that demonstrates the drug can change outcomes decisively when the evidence is strong enough.

What holds the field back, according to the review, is not a shortage of hypotheses but a shortage of certainty. The overall inconsistency in review certainty remains a barrier to integrating N-acetylcysteine into clinical guidelines, because guideline committees require consistent, high-quality evidence before recommending a therapy for new indications. A landscape in which half of the top-tier reviews support repurposing while a large share of all reviews are critically low in quality gives committees little to work with. The redundancy documented by the CCA analysis compounds the problem, since overlapping syntheses can create an illusion of consensus where the underlying trial base is thin, heterogeneous, or outdated.

The authors argue that the way forward requires precision medicine approaches to guide future clinical implementation and research. Rather than testing N-acetylcysteine as a one-size-fits-all intervention, future trials could stratify patients by oxidative stress burden, genetic determinants of glutathione metabolism, or disease subtype, identifying the populations most likely to benefit. Such targeting could also resolve some of the heterogeneity that has plagued meta-analyses, in which mixed patient groups and variable dosing regimens make pooled estimates difficult to interpret. By delineating research priorities and highlighting new avenues, the review functions as both a map of what is known and a checklist of what must be done before the drug’s promise can be translated into practice.

For clinicians and patients, the immediate takeaway is one of cautious patience. N-acetylcysteine remains a valuable and well-established therapy in its approved roles, particularly in airway disease and acetaminophen toxicity, and its safety record makes further study attractive. But the new analysis makes clear that the sprawling literature celebrating its potential has outpaced the rigorous trials needed to confirm it. The review’s contribution is to convert that sprawling enthusiasm into an organized agenda: prioritize the five highest-potential domains, replace redundant reviews with well-designed primary trials, and apply the same methodological scrutiny to repurposing evidence that regulators demand for novel drugs. If that agenda is followed, a molecule first synthesized in the nineteenth century may yet find a second, better-evidenced life in twenty-first-century medicine.

Subject of Research: Drug repurposing potential and evidence quality of N-acetylcysteine across clinical domains

Article Title: Repurposing landscape of N-acetylcysteine: A scoping review of reviews toward clinical translation

Article References: Al Alaili, M. K. (2026). Repurposing landscape of N-acetylcysteine: A scoping review of reviews toward clinical translation. PLOS One, 21(10), e0360434. https://doi.org/10.1371/journal.pone.0360434

Image Credits: AI Generated

DOI: 10.1371/journal.pone.0360434

Keywords: N-acetylcysteine, drug repurposing, scoping review, oxidative stress, systematic reviews, glutathione, evidence quality, research waste, psychiatry, reproductive medicine, nephrology, toxicology