The scientific understanding of hydrogen peroxide (H2O2) has undergone several paradigm shifts. Research over the last 30 years has shown that it has only harmful effects. There is now a scientific consensus that endogenously formed H2O2 molecule is a cellular messenger in cell signaling at physiological (low) concentrations, an active and positive player in all phases of wound healing, a mediator of many metabolic processes, involved in the regulation of apoptosis, gene transcription, can modify protein function, localization and activity through oxidative post-translational modification. Elevated tissue levels are always an indicator of some disease state (tumor, inflammation, etc.), while high concentrations of oxidative effects generate further damage. Catalase is present in all cells in variable concentrations and is a sensitive modulator of H2O2 concentration. H2O2 can exert its bactericidal effect only on bacteria containing catalase. Routine wound and tissue cleansing with 3% H2O2 solution in patient care dissolves clots and removes dead cells preventin thereby infection. However, it induces severe oxidative stress and tissue damage. It is classified as an "old antiseptic" according to European guidelines and is no longer recommended for the treatment of acute or chronic wounds due to a number of adverse properties. However, lipid-stabilized 1% H2O2 cream can be used for wound treatment. Among complementary therapies that have gained increasing attention in response to the rising prevalence of bacterial antibiotic resistance, hyperbaric oxygen therapy and medical honey-impregnated dressings act, at least in part, because they contain H2O2. Recent research aims at supporting tissue redox balance from the antioxidant enzyme side by developing biomimetic and nanoenzyme systems that can fill the missing catalase function for H2O2. Overall, among the reactive oxygen species, the endogenous H2O2 molecule has highly active physiological functions, while for routine exogenous administration, a less harmful concentration or a stabilized formula is recommended. Orv Hetil. 2026; 167(3): 119-128.