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Simple Summary: Among the most malignant cancers, oral squamous cell carcinoma stands out as the most common malignant head and neck tumor, with high prevalence around the world, mainly in developing countries. Despite advances in the field of treatment, the prognosis of patients with oral squamous cell carcinoma remains poor, with a 5-year survival rate of less than 50%. Aiming to overcome the limitations of currently existing therapies, the present work proposes to investigate the potential of photodynamic therapy with methylene blue or toluidine blue. We found higher incorporation rates of both drugs in tumor cells compared to the non-tumor cell line. In addition, photodynamic therapy increases cytotoxic activity of both compounds for all lineages evaluated. Moreover, combination of both compounds promotes a synergistic effect on the viability of oral squamous cell carcinoma, opening up possibilities for the use of photodynamic therapy against other cancer cell types using combinations of these photosensitizers. Among the most malignant cancers, oral squamous cell carcinoma (OSCC) stands out as the most common malignant head and neck tumor. Despite advances in the field of treatment, the prognosis of patients with OSCC remains poor. Aiming to overcome the limitations of the currently existing therapies against OSCC, the present work aims to investigate the potential of photodynamic therapy (PDT) with phenothiazine derivatives used alone or in combination. The incorporation of methylene blue (MB) and toluidine blue (TB) was evaluated in OSCC cell lines (HSC-3 and SCC-9) and a nontumor cell line (Hfib). Both compounds exhibited concentration and time-dependent incorporation, with higher rates observed in tumor cells. Regarding dark-phase cytotoxic activity, SCC-9 cells were the most sensitive cell line with an IC50 value of 362.6 µM and 41.4 µM for MB and TB, respectively. Using PDT, all lineages showed greater sensitivity, presenting lower IC50 values when compared to the dark phase values. The combination index values of 0.69 (dark phase) and 0.73 (clear phase) associated with concave isobolograms, in both phases, revealed that MB and TB have synergistic effects when combined against SCC-9 cells. These findings suggest that MB or TB assisted with PDT holds promise for OSCC treatment. [ABSTRACT FROM AUTHOR]