Molecular hydrogen (H₂) was believed to be inert and nonfunctional in mammalian cells. We overturned this concept by demonstrating that H₂ reacts with highly reactive oxidants such as hydroxyl radical (•OH) and peroxynitrite (ONOO
) inside cells

H₂ has several advantages exhibiting marked effects for medical applications: it is mild enough neither to disturb metabolic redox reactions nor to affect signaling by reactive oxygen species. Therefore, it should have no or little adverse effects. H₂ can be monitored with an H₂-specific electrode or by gas chromatography. H₂ rapidly diffuses into tissues and cells to exhibit efficient effects.

Thus, we proposed the potential of H₂ for preventive and therapeutic applications. There are several methods to ingest or consume H₂: inhaling H₂ gas, drinking H₂-dissolved water (H₂-water), injecting H₂-dissolved saline (H₂-saline), taking an H₂ bath, or dropping H₂-saline onto the eyes.

Recent publications revealed that, in addition to the direct neutralization of highly reactive oxidants, H₂ indirectly reduces oxidative stress by regulating the expression of various genes. Moreover, by regulating gene expression, H₂ functions as an anti-inflammatory, antiallergic, and antiapoptotic molecule, and stimulates energy metabolism. In addition to growing evidence obtained by model animal experiments, extensive clinical examinations were performed or are under way.

Since most drugs specifically act on their specific targets, H₂ seems to differ from conventional pharmaceutical drugs. Owing to its great efficacy and lack of adverse effects, H₂ has potential for clinical applications for many diseases.