© 2019, Springer-Verlag GmbH Germany, part of Springer Nature.Background: Photobiomodulation (PBM) has shown efficacy in preventing and treating cancer therapy-induced mucositis and dermatitis. However, there is contradictory information regarding the effect of PBM on (pre)malignant cells, which has led to questions regarding the safety of this technique. We address this issue using an orthotopic mouse model (Cal-33) with human squamous cell carcinoma of the oral cavity. Methods: Mice with actively growing orthotopic Cal-33 head and neck carcinoma tumors were divided into 4 groups: control, PBM only, radiation therapy (RT) only, and PBM + RT. We performed three experiments: (1) PBM at 660 nm, 18.4 J/cm2, and 5 RT × 4 Gy doses delivered daily; (2) PBM at 660 nm, 18.4 J/cm2, and 1 × 15 Gy RT; and (3) PBM at 660 nm + 850 nm, 45 mW/cm2, 3.4 J/cm2, and 1 × 15 Gy RT. Mice were weighed daily and tumor volumes were evaluated by IVIS. Survival time was also evaluated. Results: Animals treated with RT survived significantly longer and had significantly smaller tumor volume when compared with the control and PBM-only treatment groups. No significant differences were noted between the RT alone and PBM + RT groups in any of the experiments. Conclusion: Our results suggest that PBM at the utilized parameters does not provide protection to the tumor from the killing effects of RT.