Clinical use of ozone therapy: a critical appraisal of evidence, safety, and methodological quality
Ozone therapy is frequently promoted as a versatile and minimally invasive medical intervention purported to exert a broad range of biological effects, including anti-inflammatory, antimicrobial, immunomodulatory, and regenerative actions. These claims have led to its application across diverse clinical domains, such as musculoskeletal disorders, dermatological conditions, chronic wounds, peripheral vascular disease, chronic inflammatory diseases, infectious pathologies, and, more recently, viral illnesses. In many private clinical settings, ozone therapy is presented as an adjunct or alternative to established treatments, sometimes accompanied by claims of efficacy that overstate the strength of the underlying evidence. Such representations stand in contrast to the principles of evidence-based medicine, which require therapeutic claims to be supported by reproducible, high-quality clinical evidence. At the same time, as detailed below, ozone therapy is legally recognised as a complementary or traditional medical procedure in a substantial number of jurisdictions, and a growing, heterogeneous body of systematic reviews and meta-analyses -several published in the last five years -reports statistically significant benefit for specific indications, alongside other syntheses reporting verylow-certainty or unsupportive evidence for others. This coexistence of supportive, inconclusive, and negative evidence has contributed to ongoing scientific and regulatory debate regarding the appropriate role of ozone therapy in contemporary clinical practice, highlighting the need for balanced evidence synthesis that distinguishes between individual indications rather than evaluating the therapy as a single entity. The present narrative critical review aims to reflect this heterogeneity rather than to characterise ozone therapy uniformly as either validated or disproven, while examining regulatory positions, the quality and direction of clinical studies across indications, historical patterns of adoption, economic and communicative factors, and reported adverse events.This article is conceived as a narrative critical review, not a systematic review with a pre-registered protocol, quantitative meta-analysis, or formal risk-of-bias synthesis performed by the present authors. In line with recommended standards for the reporting of narrative reviews (14), we clarify here the approach used to identify and select the literature discussed below. PubMed/MEDLINE, Google Scholar, and the Cochrane Library were searched using combinations of the terms "ozone therapy", "oxygen-ozone", "autohemotherapy", "ozonated", together with indication-specific terms (e.g., "low back pain", "disc herniation", "diabetic foot ulcer", "peripheral arterial disease", "osteoarthritis", "dental caries", "dermatology") and methodological filters ("systematic review", "meta-analysis", "randomized controlled trial", "umbrella review"). Regulatory and consensus documents, including national and international ozone-therapy society declarations, were also consulted to characterise the legal and institutional status of the practice. Priority was given to systematic reviews, meta-analyses, and umbrella reviews over individual small trials or case reports, and to publications from 2010 onward, supplemented by earlier landmark meta-analyses (e.g., references 5, 6) where they remain the most complete synthesis available for a given indication. Rather than adjudicating primary studies directly, we relied on the risk-of-bias, GRADE, and AMSTAR-2 judgments reported within these secondary sources. This approach, while allowing broader synthesis across ozone's multiple administration modalities (systemic autohemotherapy, intradiscal or periarticular injection, topical application, rectal insufflation, and gas bathing), does not eliminate selection subjectivity inherent to narrative reviews, and no formal database search log, PRISMA flow diagram, or explicit inclusion/exclusion criteria were applied; this is acknowledged here as a methodological limitation of the present work.From a regulatory perspective, ozone occupies a problematic position. The U.S. Food and Drug Administration (FDA) explicitly classifies ozone as a toxic gas with no proven medical utility in specific, adjunctive, or preventive therapy, emphasising that concentrations required to achieve germicidal effects are incompatible with human safety, given ozone's strong oxidative properties and its capacity to damage cellular membranes, proteins, and lipids. In marked contrast, ozone therapy is legally recognised as a complementary or traditional medical procedure in a number of countries and jurisdictions, including Greece (regulated since 1991), Ukraine (since 2001), Italy, Spain, Portugal, Turkey, Russia, Germany, China, Cuba, Mexico, Brazil, and the United Arab Emirates, among others, as documented in the consensus Madrid Declaration on Ozone Therapy issued by the International Scientific Committee of Ozone Therapy (ISCO3) (1). The purposes for which it is authorised vary by jurisdiction: in Cuba, ozone therapy is institutionally embedded within the national Natural and Traditional Medicine system and is administered in hospital-based centres, most prominently for diabetic foot ulcers, peripheral vascular insufficiency, and chronic inflammatory conditions, under standardised dosing and contraindication protocols; in several European countries, regulation has instead proceeded at the regional or professional-association level, permitting use by licensed physicians as a complementary adjunct rather than as a first-line therapy. In parts of the United States, ozone therapy may be practised under general health-freedom or non-allopathic-medicine statutes rather than under a specific therapeutic indication. This heterogeneity illustrates that legal permission reflects jurisdiction-specific regulatory philosophy and does not, by itself, constitute a determination of clinical efficacy. Biologically, ozone is a highly reactive oxidant. While proponents often invoke hormetic or indirect antioxidant mechanisms, such effects remain incompletely substantiated. No fully validated pharmacokinetic or dose-response model yet reconciles ozone's oxidative reactivity with a consistent, generalisable therapeutic benefit across the heterogeneous modalities in which it is applied, although, as discussed below, mechanistic and clinical evidence for specific effects (e.g., growth-factor induction in wound healing, rheological changes in ischaemic tissue) has accumulated for particular indications.Ozone therapy is not a single intervention but a heterogeneous family of modalities -systemic autohemotherapy, intradiscal or periforaminal injection, topical application, rectal insufflation, and gas bathing -applied across markedly different clinical contexts. The level and direction of the available evidence differ substantially by modality and indication, and general conclusions about "ozone therapy" as an undifferentiated category risk obscuring these differences.For several indications, systematic reviews report low or very low certainty evidence that does not support a clinical recommendation. In dentistry, a systematic review and meta-analysis concluded that the available evidence for ozone in the treatment of dental caries is of very low certainty (2). In dermatology, a comprehensive review reported that studies addressing acne, ulcerations, dermatitis, and herpes are largely preliminary and methodologically inconsistent, without standardised protocols (3). In musculoskeletal medicine, an umbrella review focusing on knee osteoarthritis found that all included systematic reviews were rated as critically low quality according to the AMSTAR-2 tool (4).For other indications, however, systematic reviews and meta-analyses -several from the last five yearsreport statistically significant benefit, albeit with important caveats. For chronic low back pain due to lumbar disc herniation, an early systematic review and meta-analysis of randomised controlled trials concluded that percutaneous ozone injection yielded positive results and low morbidity, while cautioning that this conclusion rested on a small number of studies with methodological limitations and possible publication bias (5). A larger meta-analysis of 12 studies (approximately 8,000 patients) reported mean improvements in pain and Oswestry Disability Index scores comparable to those achieved with surgical discectomy, alongside a low complication rate (0.064%) (6). A more recent systematic review and meta-analysis found that ultrasound-guided periforaminal ozone infiltration produced treatment success rates at six months that were superior to those of transforaminal epidural steroid injection (7). These studies are nonetheless predominantly small, frequently nonblinded or quasi-experimental, and heterogeneous in ozone concentration, injection technique, and comparator, which limits the strength and generalisability of the conclusions.For chronic wounds, a systematic review of seven studies (506 patients, mostly with diabetic foot ulcers) found a consistent positive healing effect of topical ozone with no reported adverse events, while noting that the evidence derived largely from non-randomised designs and calling for further randomised controlled trials (8).A systematic review restricted to randomised controlled trials similarly found that ozone significantly improved wound area and lowered amputation rates specifically for diabetic foot ulcers, while noting that evidence for other refractory wound types remained insufficient for meta-analysis (9). A 2024 meta-analysis of 11 studies (960 patients) found that ozone therapy significantly reduced ulcer size, shortened healing time, decreased hospital length of stay, and reduced amputation rates compared with standard treatment, while explicitly noting that its effect did not differ from standard treatment for achieving complete ulcer resolution -a nuanced finding indicating partial rather than unequivocal benefit (10).For peripheral arterial disease and critical limb ischaemia, a narrative review of in vitro, animal, and clinical studies concluded that oxygen-ozone therapy improves tissue perfusion, glycaemic control, and rheology, reduces amputation risk in diabetic foot complications, and may lower treatment costs by approximately one quarter compared with standard antibiotic therapy, with no adverse events reported across the included clinical trials; the authors nonetheless emphasised that the underlying clinical evidence derives predominantly from small, non-randomised studies lacking long-term outcome data, and called explicitly for larger controlled trials (11).Taken together, recurring methodological limitations across both the negative and the positive syntheses include small sample sizes, inadequate randomisation and blinding (difficult to achieve given the gas's detectable odour and the procedural nature of most interventions), heterogeneous outcome measures, and the possibility of publication bias favouring positive results. Importantly, the current literature more often reflects an absence of high-certainty evidence than definitive proof of either efficacy or inefficacy -a distinction that is central to interpreting clinical uncertainty within an evidence-based framework, and one that argues against treating ozone therapy as a monolithic, uniformly disproven, or uniformly validated intervention.Ozone was isolated in 1840 by Christian Friedrich Schönbein, who first described its characteristic pungent odour and recognised its strong oxidative properties. In the late nineteenth and early twentieth centuries, ozone attracted intermittent medical interest, primarily due to its presumed disinfectant potential. During the first half of the twentieth century, particularly in military and emergency medical contexts, ozone was empirically applied to the treatment of infected wounds and ulcers, at a time when antibiotics were either unavailable or in limited supply. The widespread introduction of penicillin and subsequent generations of antibiotics during the midtwentieth century substantially reduced clinical interest in ozone as an antimicrobial treatment, as pharmacological therapies offered more predictable efficacy, standardised dosing, and a rapidly expanding evidence base. These early applications preceded the development of modern clinical trial methodology and were conducted without standardised protocols, control groups, or systematic assessment of efficacy and safety.Unlike medical technologies such as hyperbaric oxygen therapy, dialysis, or extracorporeal membrane oxygenation -which emerged from experimental research and subsequently underwent rigorous clinical validation, regulatory scrutiny, and institutional adoption -ozone therapy has followed a markedly more fragmented trajectory, achieving formal institutional integration in some health systems (e.g., Cuba, as noted above) while remaining largely outside academic medicine and university-based training in others, including the United States. The consolidation of evidence-based medicine during the 1990s reinforced the requirement that therapeutic interventions demonstrate efficacy through well-designed randomised controlled trials and systematic evidence synthesis; within this evolving framework, ozone therapy's continued reliance on privatesector training programmes, specialised courses, and dedicated international associations, operating substantially outside conventional regulatory and educational structures, has limited its broader integration even where isolated indications now have supportive systematic evidence.In addition to biomedical considerations, several contextual factors discussed in the health-communication and risk-perception literature may contribute to the persistence of certain medical practices independent of the strength of the underlying clinical evidence. A well-documented phenomenon in judgment and decision-making research is the affect heuristic, whereby individuals substitute an immediate emotional impression for a more effortful analytic assessment of risk and benefit, particularly when information is incomplete, technical, or ambiguous; experimental work has repeatedly shown an inverse relationship between perceived benefit and perceived risk that tracks the valence of the underlying affective association rather than objective probability (15). Within health communication research specifically, the framing and affective tone of the language used to describe a condition, or intervention has been shown to shape patients' risk perceptions and treatment preferences (16). An often-overlooked contributor to the appeal of ozone therapy is linguistic association: in public discourse, the term "ozone" is strongly linked to the atmospheric ozone layer, commonly portrayed as a protective shield against harmful ultraviolet radiation. While no study has, to our knowledge, directly tested this specific semantic association for the word "ozone", the broader affect-heuristic literature provides a plausible psychological mechanism by which such positive framing could foster an intuitive perception of the therapy as inherently protective, independent of clinical evidence, and could accordingly shape patient expectations and public understanding of the intervention (15,16).Commercial market-research reports estimate that the global market for