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Photobiomodulation therapy (PBMT) uses light to stimulate cells. The molecular basis of the effects of PBMT is being unveiled, but it is stated that the cytochrome-c oxidase enzyme in mitochondria, a photon acceptor of PBMT, contributes to an increase in ATP production and modulates the reduction and oxidation of electron carriers NADH and FAD. Since its effects are not fully understood, PBMT is not used on tumors. Thus, it is interesting to investigate if its effects correlate to mitochondrial metabolism and if so, how it could be linked to the optical redox ratio (ORR), defined as the ratio of FAD/(NADH + FAD) fluorescences. To that end, fibroblasts (HDFn cell line) and oral squamous cell carcinoma (SCC-25 cell line) were irradiated with a light source of 780 nm and a total dose of 5 J/cm2, and imaged by optical microscopy. PBMT down-regulated the SCC-25 ORR by 10%. Furthermore, PBMT led to an increase in ROS and ATP production in carcinoma cells after 4 h, while fibroblasts only had a modest ATP increase 6 h after irradiation. Cell lines did not show distinct cell cycle profiles, as both had an increase in G2/M cells. This study indicates that PBMT decreases the redox state of oral cancer by possibly increasing glycolysis and affects normal and tumor cells through distinct pathways. To our knowledge, this is the first study that investigated the effects of PBMT on mitochondrial metabolism from the initiation of the cascade to DNA replication. This is an essential step in the investigation of the mechanism of action of PBMT in an effort to avoid misinterpretations of a variety of combined protocols.