Hydrogen peroxide (H2O2) is a very pale blue liquid that appears colourless in a dilute solution. H2O2 is slightly more viscous than water and is a weak acid. H2O2 is unstable and slowly decomposes in the presence of light. It has strong oxidizing properties and is, therefore, a powerful bleaching agent that is mostly used for bleaching paper. H2O2 has also found use as a disinfectant and as an oxidizer. H2O2 in the form of carbamide peroxide is widely used for tooth whitening (bleaching), both in professionally- and in self-administered products. H2O2 is a well-documented component of living cells and is a normal metabolite of oxygen in the aerobic metabolism of cells and tissues. A total of 31 human cellular H2O2 generating enzymes has been identified so far (PMID: 25843657). H2O2 plays important roles in host defence and oxidative biosynthetic reactions. At high levels (>100 nM) H2O2 is toxic to most cells due to its ability to non-specifically oxidize proteins, membranes and DNA, leading to general cellular damage and dysfunction. However, at low levels (<10 nM), H2O2 functions as a signalling agent, particularly in higher organisms. In plants, H2O2 plays a role in signalling to cause cell shape changes such as stomatal closure and root growth. As a messenger molecule in vertebrates, H2O2 diffuses through cells and tissues to initiate cell shape changes, to drive vascular remodelling, and to activate cell proliferation and recruitment of immune cells. H2O2 also plays a role in redox sensing, signalling, and redox regulation (PMID: 28110218). This is normally done through molecular redox “switches” such as thiol-containing proteins. The production and decomposition of H2O2 are tightly regulated (PMID: 17434122). In humans, H2O2 can be generated in response to various stimuli, including cytokines and growth factors. H2O2 is degraded by several enzymes including catalase and superoxide dismutase (SOD), both of which play important roles in keeping the amount of H2O2 in the body below toxic levels. H2O2 also appears to play a role in vitiligo. Vitiligo is a skin pigment disorder leading to patchy skin colour, especially among dark-skinned individuals. Patients with vitiligo have low catalase levels in their skin, leading to higher levels of H2O2. High levels of H2O2 damage the epidermal melanocytes, leading to a loss of pigment (PMID: 10393521). Accumulating evidence suggests that hydrogen peroxide H2O2 plays an important role in cancer development. Experimental data have shown that cancer cells produce high amounts of H2O2. An increase in the cellular levels of H2O2 has been linked to several key alterations in cancer, including DNA changes, cell proliferation, apoptosis resistance, metastasis, angiogenesis and hypoxia-inducible factor 1 (HIF-1) activation (PMID: 17150302, 17335854, 16677071, 16607324, 16514169). H2O2 is found in most cells, tissues, and biofluids. H2O2 levels in the urine can be significantly increased with the consumption of coffee and other polyphenolic-containing beverages (wine, tea) (PMID: 12419961). In particular, roasted coffee has high levels of 1,2,4-benzenetriol which can, on its own, lead to the production of H2O2. Normal levels of urinary H2O2 in non-coffee drinkers or fasted subjects are between 0.5-3 uM/mM creatinine whereas, for those who drink coffee, the levels are between 3-10 uM/mM creatinine (PMID: 12419961). It is thought that H2O2 in urine could act as an antibacterial agent and that H2O2 is involved in the regulation of glomerular function (PMID: 10766414).