Molecular Hydrogen in Health and Medicine
Molecular hydrogen (H₂) is a diatomic gas that, in biological systems, functions as a selective antioxidant preferentially neutralizing the most cytotoxic reactive oxygen species while preserving physiologically important radicals involved in normal cell signaling[^c1][^c2]. This selectivity distinguishes it from conventional antioxidants and has made it the subject of extensive biomedical research since the landmark 2007 study by Ohsawa et al. in Nature Medicine[^c3]. Beyond direct radical scavenging, H₂ activates the Nrf2 transcription factor to upregulate endogenous antioxidant enzymes, inhibits the NF-κB pathway to reduce pro-inflammatory cytokines, and protects mitochondrial function[^c4][^c5]. By mid-2026, over 1,500 basic research papers and 60 clinical trials had been published on hydrogen medicine[^c29]. Recent work has identified the Rieske iron–sulfur protein (RISP) in mitochondrial complex III as a primary molecular target of H₂, triggering a mitohormetic adaptive response—a significant advance in understanding how trace amounts of H₂ produce marked biological effects[^c13]. In 2024, oxidized heme (Heme-Fe(III)-OH) was identified as an additional core molecular target, providing a unified explanation for H₂'s selective antioxidant mechanism[^c22].
The biological effects of H₂ have been investigated across more than 38 disease conditions, including ischemia-reperfusion injury, neurodegenerative disorders, cardiovascular disease, metabolic syndrome, and inflammatory conditions[^c6]. Clinical research in 2026 continued to expand across multiple therapeutic areas. In cardiovascular medicine, a pioneering injectable hydrogel encapsulating living photosynthetic bacteria that produces H₂ on demand when exposed to light demonstrated reduced infarct size and improved cardiac function after myocardial ischemia-reperfusion injury in rodent and porcine models[^c28]. A 2026 randomized controlled trial showed that combined inhaled nitric oxide and molecular hydrogen improved dyspnea, fatigue, exercise tolerance, and oxidative status in post-COVID syndrome patients[^c30]. Low-concentration hydrogen inhalation was shown to improve cognitive function in elderly women with mild cognitive impairment, shifting MMSE scores from the suspected dementia range to the normal range[^c18]. A closed-loop glucose-responsive wound dressing combining nitric oxide and molecular hydrogen demonstrated complete wound closure in diabetic mouse models, representing a novel precision gas medicine approach[^c20]. The first Phase III multicenter randomized controlled trial of supersaturated hydrogen-rich water for weight management (the HOPE trial) began recruitment in April 2026[^c14].
A 2026 systematic review of 45 studies evaluated molecular hydrogen therapy across musculoskeletal conditions including osteoarthritis, rheumatoid arthritis, and exercise-induced muscle damage, reporting symptomatic benefits but noting that GRADE evidence was rated as low or very low across all trials[^c23]. In critical care research, inhaled hydrogen improved survival and preserved cognitive function in sepsis models through Nrf2/HO-1 signaling and mitochondrial quality control[^c24]. Cardiovascular studies expanded with a Keio University trial reporting that hydrogen-rich water enhanced heart rate variability at rest in healthy adults, and a prospective observational study in older adults associated routine HRW consumption with improved lower extremity function[^c25][^c26]. In dermatology, a double-blind randomized trial found that hydrogen-rich water reduced pain and itching in keloid patients, and a mouse study showed that continuous H₂ administration delayed UVB-induced skin carcinogenesis[^c27]. In oncology, a systematic review of 27 studies reported significantly improved progression-free survival in advanced NSCLC patients receiving adjunctive H₂ inhalation[^c17].
Administration routes include inhalation of hydrogen gas, drinking hydrogen-rich water, hydrogen baths, and oral solid supplements, each with distinct pharmacokinetic characteristics[^c7]. Conventional delivery routes face critical limitations in stability, bioavailability, and targeted delivery, spurring development of advanced delivery systems including H₂-containing carriers, in situ H₂-generating materials, and externally stimulated platforms such as photo-, sono-, and electro-catalysis-based systems[^c31]. H₂ is a physiologically normal molecule produced by intestinal bacteria and has demonstrated no toxic side effects across exposures far exceeding therapeutic levels[^c8], though a 2026 safety study found that pure hydrogen inhalation causes a mild decrease in blood oxygen saturation from dilution of inspired oxygen[^c16]. In Japan, electrolyzed hydrogen water generators are certified as Class II medical devices for gastrointestinal symptom improvement, and hydrogen inhalation for post-cardiac arrest syndrome has been designated as an advanced medical therapy[^c9][^c10]. However, multiple severe explosion accidents have been documented from high-concentration hydrogen inhalers (67–100 vol%) that far exceed the verified safe threshold of 10 vol%[^c19].
Despite encouraging findings, the field faces significant challenges. Many studies rely on small sample sizes and exhibit methodological variability; a 2026 evidence-based review rated the GRADE evidence for hydrogen in musculoskeletal conditions as "low" or "very low" across all included studies[^c15]. The mechanisms underlying H₂'s biological activity at low concentrations remain incompletely understood, though the discovery of the RISP-mediated mitohormetic pathway and the identification of oxidized heme as a core molecular target have begun to close this gap[^c13][^c22]. Commercial promotion of hydrogen products has often outpaced the scientific evidence, and no national mandatory standards exist for hydrogen water concentration or quality in most countries[^c12].